docs(rfc): classify RFCs by kind via path-encoded subdirectories

Add a second axis to every RFC — its class (feature, bug-fix,
simplification, architecture, process, testing) — encoded in the path
as docs/rfc/{lifecycle}/{class}/file.md. The folder is the label, so
the closed set is enforced by structure rather than a parsed field.

Two new doc-sync gates back it:
- verify-rfc-classification: every RFC sits in a valid class folder and
  the README index lists it under the matching lifecycle→class heading.
- verify-doc-refs: every docs/*.md path cited in a packages|examples TS
  comment resolves — closes a drift class verify-md-links can't see, and
  catches the four comment refs this reorg moved.

The README gains a Classification section explaining the taxonomy and
per-class index sub-sections. A self-referential process RFC records why
the scheme is path-encoded and gated.

packages/llm-replay/src/index.ts

@@ -5,7 +5,7 @@
  * waterfall (never calls `next()`) and yields model streams reconstructed from
  * a recorded **session JSONL** fixture — so a snapshot test can boot the real
  * agent against a fixed model transcript with no API key. See
- * docs/rfc/implemented/2026-06-19-acp-snapshot-tests.md.
+ * docs/rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md.
  *
  * The fixture IS the persisted session log (`<scenario>/session.jsonl`): its
  * `assistant/chunk` events carry every {@link StreamChunk}, so grouping them by