perf(session): stat the probe parent only on Windows

`JsonlSessionPersistence.exists` treats ENOENT as absence and stats the
path's parent first, so a session directory blocked by a regular file
stays a storage fault. Only Windows needs that: it reports ENOENT rather
than ENOTDIR for `regular-file/child`, while POSIX open reports ENOTDIR
before the branch is reached.

The stat ran on every platform and every absent probe. findLog issues
four probes per project directory and the coordinator resolves an id
twice per inspect, so nearly every probe paid it. Counting fs calls
against a five-project store, loading an existing session drops from
40 open + 41 stat to 40 open + 3 stat.

.agents/notes/implemented/simplification/2026-08-28-windows-only-absent-probe-repair.i18n.yaml

@@ -0,0 +1,6 @@
+# Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
+# side as of the last confirmed-consistent state. Both languages carry equal authority;
+# after editing either side, bring the other along and re-record with:
+#   pnpm run verify-translation-pairing --write .agents/notes/implemented/simplification/2026-08-28-windows-only-absent-probe-repair.md
+2026-08-28-windows-only-absent-probe-repair.md: 55fa7bee8d6e16eb96223fa9745f95891cfdf89c
+2026-08-28-windows-only-absent-probe-repair.zh.md: 5a459cab1f3d914934945465be1d1f08e0c4530b

.agents/notes/implemented/simplification/2026-08-28-windows-only-absent-probe-repair.md

@@ -0,0 +1,40 @@
+# Agent Note: The absent-probe parent repair runs on Windows only
+
+Status: implemented
+
+English | [中文](2026-08-28-windows-only-absent-probe-repair.zh.md)
+
+## Problem
+
+`JsonlSessionPersistence.exists` treats ENOENT as absence, and before returning false it stats the path's parent so a session directory blocked by a regular file surfaces as a storage fault rather than as a missing session. Windows needs that: it reports ENOENT, not ENOTDIR, for `regular-file/child`. POSIX does not — `open(2)` specifies ENOTDIR when a component of the path prefix is not a directory, which the repair's own guard comment already recorded.
+
+The stat ran on every platform and on every absent probe. `findLog` issues four probes per project directory — two rejecting the legacy flat-file layout, one for the opposite encoding, one for the log itself — and the coordinator resolves an id twice per `inspect`: once in `prepareCore`, once in `isPreparedSourceCurrent`, which runs even when the prepared source is cached. All but one directory answers absent, so nearly every probe paid the extra stat.
+
+## Decision
+
+The repair is reached only under `process.platform === 'win32'`, matching the platform dispatch `materialize` already uses. POSIX keeps the ENOTDIR that `open` itself reports.
+
+## Testing
+
+Counting `node:fs/promises` calls through the suite's existing module mock, against a store with five project directories — the layout of a real `~/.dsh/sessions`:
+
+| Operation | Before | After |
+|---|---|---|
+| `load` an existing session | 40 open + 41 stat | 40 open + 3 stat |
+| `load` an absent id | 20 open + 20 stat | 20 open + 0 stat |
+
+The package's 242 tests pass unchanged and per-file coverage is identical; the new branch carries the same `v8 ignore` marker as the sibling platform dispatch.
+
+That POSIX never reaches the repair was confirmed directly rather than taken from the comment: opening `regular-file/child` and `regular-file/child/deeper` reports ENOTDIR on macOS, while only a genuinely missing directory reports ENOENT.
+
+## Alternatives considered
+
+**Keep the stat on every platform as defense in depth.** Rejected: on POSIX it can only confirm what `open` already reported, so it detects no fault the caller would otherwise miss — it doubles the syscalls of every absent probe to re-derive a known answer.
+
+**Remove the repair outright and let Windows report absence.** Rejected: it exists so a session directory blocked by a regular file stays a storage fault instead of reading as a missing session, which is the fail-loud stance the backend takes elsewhere.
+
+## Consequences
+
+Path resolution keeps its full cost in `open` calls: four per project directory per lookup, twice per `inspect`. This removes the stat half only, and the remaining cost still grows with the number of project directories rather than with the number of sessions.
+
+Collapsing the scan needs an id-to-path index, which first requires deciding what happens to the per-lookup legacy-artifact guard that `rejects a compressed obsolete flat-file artifact during targeted lookup` pins: that test writes its artifact after `list()` has memoized the root encoding check, so the guard covers store mutation after memoization, and an index hit would return before reaching it. One `readdir` per project directory, which would supply both the id-to-path map and that guard without a cache, measured no faster than this change, so any further win requires memoization and its invalidation.

.agents/notes/implemented/simplification/2026-08-28-windows-only-absent-probe-repair.zh.md

@@ -0,0 +1,40 @@
+# Agent Note: 缺失探测的父目录修复只在 Windows 上执行
+
+Status: implemented
+
+[English](2026-08-28-windows-only-absent-probe-repair.md) | 中文
+
+## Problem
+
+`JsonlSessionPersistence.exists` 把 ENOENT 视为「不存在」，并在返回 false 之前 stat 一次路径的父目录，好让「被普通文件挡住的会话目录」暴露成存储故障，而不是一个不存在的会话。Windows 需要这一步：它对 `regular-file/child` 报的是 ENOENT 而不是 ENOTDIR。POSIX 不需要——`open(2)` 规定路径前缀中有非目录组件时报 ENOTDIR，这一点该修复自己的守卫注释早已写明。
+
+而这次 stat 在所有平台、每一次缺失探测上都会执行。`findLog` 对每个 project 目录发四次探测——两次拒绝 legacy 扁平文件布局、一次查相反编码、一次查日志本身——而 coordinator 每次 `inspect` 要解析两遍 id：一遍在 `prepareCore`，一遍在 `isPreparedSourceCurrent`，后者即使 prepared source 已缓存也照跑。除拥有该会话的那个目录外，其余全部回答「不存在」，所以几乎每次探测都付了这次多余的 stat。
+
+## Decision
+
+该修复现在只在 `process.platform === 'win32'` 下可达，与 `materialize` 既有的平台分派写法一致。POSIX 保留 `open` 自己报出的 ENOTDIR。
+
+## Testing
+
+借用测试套件既有的模块 mock 统计 `node:fs/promises` 调用，对照一个五 project 目录的存储——即真实 `~/.dsh/sessions` 的布局：
+
+| 操作 | 改动前 | 改动后 |
+|---|---|---|
+| `load` 一个已存在的会话 | 40 open + 41 stat | 40 open + 3 stat |
+| `load` 一个不存在的 id | 20 open + 20 stat | 20 open + 0 stat |
+
+该包的 242 个测试原样通过，单文件覆盖率不变；新增分支带的 `v8 ignore` 标记与相邻的平台分派一致。
+
+「POSIX 永远到不了这处修复」是直接验证的，而非采信注释：在 macOS 上打开 `regular-file/child` 与 `regular-file/child/deeper` 都报 ENOTDIR，只有真正缺失的目录才报 ENOENT。
+
+## Alternatives considered
+
+**所有平台都保留这次 stat，作为纵深防御。** 否决：在 POSIX 上它只能确认 `open` 已经报出的结论，检测不到任何调用方本会漏掉的故障——代价是把每次缺失探测的系统调用翻倍，只为重新推导一个已知答案。
+
+**直接删掉该修复，让 Windows 报「不存在」。** 否决：它的存在正是为了让「被普通文件挡住的会话目录」保持为存储故障，而不是读成一个不存在的会话，这与该 backend 别处一贯的 fail-loud 立场一致。
+
+## Consequences
+
+路径解析在 `open` 调用上的开销原样保留：每次查找每个 project 目录四次，每次 `inspect` 两轮。本次只削掉了 stat 那一半，剩余开销仍随 project 目录数增长，而不随会话数增长。
+
+要压掉这轮扫描需要一份 id→path 索引，而它首先要求决定：`rejects a compressed obsolete flat-file artifact during targeted lookup` 钉住的那条「每次查找都做的 legacy 产物守卫」该如何安置。该测试是在 `list()` 已经记忆化 root 编码检查之后才写入产物的，所以这条守卫覆盖的是记忆化之后的存储变动，而索引命中会在到达它之前就返回。「每个 project 目录一次 `readdir`」本可以同时提供 id→path 映射和这条守卫且无需缓存，但实测并不比本次改动更快，所以更进一步的收益必然要引入记忆化及其失效处理。

packages/session/session-persistence-jsonl/src/index.ts

@@ -953,11 +953,14 @@ export class JsonlSessionPersistence extends SessionPersistence implements Persi
     } catch (error) {
       // Only ENOENT means absent. A permission/I/O error must surface rather
       // than letting load or collision checks proceed under false absence.
-      // Windows reports ENOENT, not ENOTDIR, for `regular-file/child`; verify
-      // the immediate parent so a blocked session directory remains a storage fault.
       /* v8 ignore else -- Windows reports file-valued parents as ENOENT; POSIX covers direct ENOTDIR. */
       if (isENOENT(error)) {
-        await this.assertLogParentAllowsAbsence(path)
+        // Windows reports ENOENT, not ENOTDIR, for `regular-file/child`, so it
+        // alone verifies the immediate parent to keep a blocked session
+        // directory a storage fault. POSIX open already reported ENOTDIR before
+        // this point, where the extra stat would only cost a syscall per probe.
+        /* v8 ignore next -- native Windows coverage exercises this platform dispatch; POSIX reports ENOTDIR from open */
+        if (process.platform === 'win32') await this.assertLogParentAllowsAbsence(path)
         return false
       }
       /* v8 ignore next -- Windows repairs ENOTDIR from ENOENT above; POSIX covers direct ENOTDIR. */