docs(session): renumber session-persistence ADR reference 0016→0018

master's pnpm migration claimed ADR 0016 (0016-pnpm-over-yarn), which
collides with this stack's session-persistence ADR. Renumbering the
session-persistence ADR to 0018 (turn-enclosure stays 0017); update the
json.ts module-doc reference accordingly.

packages/session/src/json.ts

@@ -1,7 +1,7 @@
 /**
  * JSON-serializability validation for session event data.
  *
- * The session event log is the durable source of truth (ADR 0003/0016): every
+ * The session event log is the durable source of truth (ADR 0003/0018): every
  * `event.data` must round-trip losslessly through JSON so any persistence
  * backend can store and reload it byte-identically. This invariant belongs to
  * the log itself — `Session.append` enforces it at the source, so a