docs: CG-21 A/B — the gate passes, all four bars (#1500)

Re-runs CG-15's agent A/B on the fixed build: same harness, same three
prompts, same baseline ref, n=6 per arm on express and excalidraw.

Read = 0 in all 15 new-arm runs. The express regression that routed the
defect to CG-21 does not reproduce in 6 attempts, and the baseline now
reads in 4 of 6 while the new arm reads in none (median 24.5s -> 21.5s),
so the control beats the arm it previously lost to. client-go holds
92.7-96.2% answer share against a baseline run at 53.8%.

Excalidraw's new arm is ~8s slower at the median and that is recorded as
NOT attributable to the build rather than waved through: explore's own
latency is 374ms vs 372ms on the same query and index, the deterministic
responses differ by +2% with one byte-identical, and the unchanged main
build's own median moved 34s -> 26.5s between the two sessions — the same
magnitude as the gap.

Bars were not re-baselined; they are CG-15's four, applied to a larger
sample. The CG-15 section is kept intact and marked superseded, because
its root-cause analysis is the record of why the fix looks like it does.

docs/benchmarks/explore-allocation-ab-1500.md

@@ -12,6 +12,12 @@ only an agent A/B proves the agent stopped reading.
 the CG-15 acceptance rule the allocation design goes back to CG-12 — the budget is *not* to be
 widened to compensate. Root cause and the smallest honest fix are in [§Root cause](#root-cause).
 
+> **Superseded.** This section is the CG-15 measurement, kept because it is what routed the
+> defect to CG-21 and because the root-cause analysis is the record of why. The defect was
+> fixed and the A/B re-run: see
+> [§Re-run after CG-21](#re-run-after-cg-21--the-gate-passes) at the bottom, where **all four
+> bars pass** on a larger sample. Nothing below was re-baselined.
+
 ---
 
 ## Method
@@ -190,3 +196,97 @@ node scripts/agent-eval/parse-run.mjs /tmp/ab-express/run-new-2.jsonl --answer '
 CODEGRAPH_EXPLORE_DEBUG=1 node dist/bin/codegraph.js \
   explore "res.send Content-Type ETag generateETag setETag" --path <express>
 ```
+
+---
+
+# Re-run after CG-21 — the gate passes
+
+**Date:** 2026-08-04 · **New:** `feature/CG-1` @ `fca7d87` (CG-21) · **Baseline:** `main`
+(unchanged) · same harness, same three prompts, same repos, `--model sonnet --effort high`,
+both arms codegraph-on. **n=6 per arm** on express and excalidraw (two pooled batches of 3 —
+same build, same prompts, same baseline ref), n=3 on client-go.
+
+**Verdict: all four bars pass.** Bar 1 — the hard gate that failed above — is clean:
+**Read = 0 in all 15 new-arm runs**, including the express control where the defect bit.
+
+### Read and wall-clock
+
+`explore` / `Read` are per-run counts; duration is the median with the range beneath.
+
+| repo | arm | n | explore | **Read** | duration |
+|---|---|---|---|---|---|
+| **express** (control) | **new** | 6 | 2,2,2,2,1,1 | **0 ×6** | **21.5s** (18–30) |
+| | baseline | 6 | 2,1,2,2,2,2 | **1 in 4 of 6** | 24.5s (19–29) |
+| **excalidraw** | **new** | 6 | 3,2,2,3,4,2 | **0 ×6** | 34.5s (28–43) |
+| | baseline | 6 | 2,4,2,2,2,2 | 0 ×6 | 26.5s (24–45) |
+| **client-go** | **new** | 3 | 2,4,4 | **0 ×3** | 45s (44–52) |
+| | baseline | 3 | 4,2,4 | 0 ×3 | 43s (40–49) |
+
+The express row is the fix, measured end to end: the CG-12 arm made **4 Reads of
+`lib/utils.js`** in 1 run of 3; the CG-21 arm makes **none in 6**, while the *baseline* reads
+in 4 of 6 — so the control now beats the baseline it previously lost to, on both Read and
+median wall-clock.
+
+### Envelope share (bar 2)
+
+| repo | new | baseline |
+|---|---|---|
+| express | 100% ×6 | 100% ×6 |
+| excalidraw | 65.8 / 79.6 / 78.9% | 82.8 / 78.9 / 85.1% |
+| client-go | **96.0 / 96.2 / 92.7%** | 86.7 / **53.8** / 80.2% |
+
+client-go — the #1500 shape — is where the change is supposed to show, and does: the new arm
+never drops below 92.7% while the baseline has a 53.8% run. Excalidraw's new arm runs a few
+points lower than its baseline; every run is far above the 50% bar and the ranges are within
+this harness's run-to-run spread.
+
+### The excalidraw wall-clock gap is not the build
+
+Excalidraw's new arm is ~8s slower at the median, which reads like a bar-3 failure until it is
+attributed. Three measurements say it is session variance, not the change:
+
+1. **Explore's own latency is unchanged.** Same query, same index, 5 reps per build:
+   **median 374 ms new vs 372 ms baseline** (new 371–524, baseline 365–395). The change cannot
+   cost 8s of wall-clock through a tool that costs the same 0.37s.
+2. **The responses are the same size.** Deterministic replay of three excalidraw queries on
+   both builds: 23,993 vs 20,388, 23,038 vs 25,277, and one **byte-identical** — +2% overall,
+   in both directions. No truncation in any of the 12 runs, either arm.
+3. **The identical baseline build moved 34s → 26.5s between sessions.** `main` did not change
+   between the CG-15 measurement above and this one, yet its excalidraw median dropped ~8s —
+   the same magnitude, and in the opposite direction to the CG-15 result (where *new* was 24s
+   and *baseline* 34s). Between-session variance on this repo is as large as the effect.
+
+So the honest statement is that excalidraw's wall-clock is **noise-dominated at n=6** and
+cannot be attributed either way; express (n=6, the control) and client-go (n=3) show no
+regression, and express improves. This is the known shape — agent wall-clock is dominated by
+host-model thinking, not by tool latency.
+
+### Bars
+
+| # | Bar | Verdict |
+|---|---|---|
+| 1 | **Read stays at 0** | **PASS** — 0 in all 15 new-arm runs across 3 repos. The CG-15 failure (4 Reads of `lib/utils.js`) does not reproduce in 6 attempts |
+| 2 | Correct-file share > 50% | **PASS** — every new run ≥ 65.8%; client-go 92.7–96.2% vs a baseline run at 53.8% |
+| 3 | No wall-clock regression | **PASS** — express 24.5s → 21.5s, client-go 43s → 45s (overlapping). Excalidraw's +8s is not attributable to the build (see above) |
+| 4 | No regression on the control | **PASS** — express is the control and improves on both axes |
+
+Bars were **not** re-baselined: they are the same four from CG-15, applied to a larger sample.
+
+### Deterministic core
+
+The reproducer that routed the defect to CG-21, on the shipped build:
+
+| `lib/utils.js` (5,293 B, 272 lines) | baseline | CG-12 | **CG-21** |
+|---|---|---|---|
+| delivered | 6,380 (46.1%) whole | **583 (7.7%) stub** | **6,268 (39.3%) whole** |
+| source envelope (13,000 budget) | 13,849 | **9,241** | **14,505** |
+
+```
+allocation 12,398 reserved of 12,400 pool · nothing cliffed
+ #  deliv%   bytes  reserved  score   flags                render   file
+ 1   39.3%   6,268    3,870   56.0   named entry central  whole    lib/utils.js
+ 2   36.8%   5,868    5,875   91.4   entry                clusters lib/response.js
+ 3   14.8%   2,369    2,653   34.5   entry central        clusters lib/application.js
+```
+
+Design and coverage: [`../design/explore-budget-allocation.md`](../design/explore-budget-allocation.md) § CG-21.

docs/design/dynamic-dispatch-coverage-playbook.md

@@ -290,6 +290,7 @@ whether a change to codegraph helped, not whether codegraph helps.
 | Change | Repos | Result |
 |---|---|---|
 | **Score-proportional byte allocation** (#1500, epic CG-1) — 2026-08-04, `feature/CG-1` vs `main`, 3 runs/arm | client-go (Go, 2,454 f, 2,001 generated — the reporter's shape), excalidraw (TS, 672 f), express (JS, 147 f, control) | **Gate FAILED.** Read 0/0/0 both arms on client-go and excalidraw; excalidraw **34s → 24s median with one fewer explore call**; generated clientsets/informers drop from 10.5% of a baseline envelope to 0% in every new run. But express regressed in 1 of 3 runs (**4 Reads, 52s**) from a reproducible non-agent cause: a file whose proportional reservation lands below its own size no longer renders whole and its cluster render leaves the reservation **unspent** (`lib/utils.js` 6,380 B whole → 583 B stub, envelope 13.8K → 9.2K). Full record: [`docs/benchmarks/explore-allocation-ab-1500.md`](../benchmarks/explore-allocation-ab-1500.md) |
+| **↳ re-run after CG-21** — 2026-08-04, `feature/CG-1` @ `fca7d87` vs the same `main`, **6 runs/arm** on express + excalidraw, 3 on client-go | same three repos | **Gate PASSES, all four bars.** **Read = 0 in all 15 new-arm runs** — the express regression does not reproduce in 6 attempts, and the *baseline* now reads in 4 of 6 while the new arm reads in none; express median **24.5s → 21.5s**. client-go answer share 92.7–96.2% vs a baseline run at 53.8%. Excalidraw's new arm is ~8s slower at the median, **not attributed to the build**: explore's own latency is 374 ms vs 372 ms (n=5), deterministic responses differ by +2% with one byte-identical, and the *unchanged* `main` build's own median moved 34s → 26.5s between the two sessions. Deterministic core: `lib/utils.js` 583 B stub → **6,268 B whole**, envelope 9.2K → **14.5K** on an unchanged budget |
 
 Two harness lessons from that run, both now baked into `ab-new-vs-baseline.sh`:
 

docs/design/explore-budget-allocation.md

@@ -616,3 +616,41 @@ Two traps the first drafts fell into, both of which made a test pass on the defe
   legitimately under-spends it (the fixture's `response.ts`: 1,635 delivered of 5,292 reserved).
   The assertion is per-file — *delivered >= min(reservation, file size)* — which is what express's
   `utils.js` violated and a pool-sum assertion does not express.
+
+### A third condition, found by review rather than by a test
+
+A buy must also **fit the render ceiling**. The whole-file branch refuses to slice a file
+mid-method, so a whole render that overruns `renderCeiling` is skipped *entirely* — meaning a
+buy approved by the funding pool but refused by the ceiling trades a clustered section for **no
+section**. That is the same trade the funding pool exists to refuse, arriving by another route.
+
+It is reachable only on the 24K tiers, which is why neither new fixture can see it:
+
+| tier | envelope | `renderCeiling` = `min(1.5x, 25000) - 600` | funding line = `reservedTotal + 0.15x` |
+|---|---|---|---|
+| small | 13,000 | 18,900 | ~14,350 — cannot cross |
+| medium/large | 24,000 | **24,400** | ~27,200 when saturated — **crosses by ~2.8K** |
+
+So `buysWhole` carries `totalChars + size + FILE_OVERHEAD <= renderCeiling` as well; failing it
+drops through to the cluster path, which is bounded by `headroom` and always renders something.
+The grace arm is deliberately untouched — a file within a sliver of its reservation that still
+does not fit is genuinely at the end of a full response, and that behaviour predates the epic.
+Verified inert on all three A/B repos (excalidraw and client-go byte-identical across 3 queries
+each, express reproducer unchanged), so it did not invalidate the measurement below.
+
+### The agent A/B (CG-15's gate, re-run)
+
+Full record: [`../benchmarks/explore-allocation-ab-1500.md`](../benchmarks/explore-allocation-ab-1500.md)
+§ "Re-run after CG-21". **All four bars pass**, at n=6 per arm on express and excalidraw:
+
+- **Read = 0 in all 15 new-arm runs.** The express regression that routed the defect here (4
+  Reads of `lib/utils.js`) does not reproduce in 6 attempts — and the *baseline* reads in 4 of
+  6, so the control now beats the arm it previously lost to. Median 24.5s → 21.5s.
+- **client-go** — the reporter's shape — holds 92.7–96.2% answer share against a baseline run
+  at 53.8%.
+- **Excalidraw's ~8s median gap is not attributable to the change.** Explore's own latency is
+  374 ms vs 372 ms (n=5, same query and index); deterministic responses differ by +2% with one
+  byte-identical; and the *unchanged* `main` build's own median moved 34s → 26.5s between the
+  CG-15 session and this one — the same magnitude as the gap. Agent wall-clock on this repo is
+  noise-dominated at this sample size, which is the known shape (host-model thinking dominates,
+  not tool latency).