feat(610): split the decision corpus into one YAML-frontmatter file per record

168 records -> docs/decisions/records/<area>/<topic>.md (163 active, 23 dirs) and
docs/decisions/archive/<area>/<topic>.md (5 archived). The filename IS the key,
so one-active-record-per-key becomes a filesystem property rather than a
validator check, and supersession becomes a `git mv`.

WHY: the monolith was a concurrency problem before an aesthetic one. A
3,900-line append target made parallel sessions collide -- PR #605 and PR #614
both hit append-vs-append conflicts during routine rebases, and hand-resolving
those inside the corpus is exactly the operation the rationale-rewrite guard
exists to police.

HOW IT IS VERIFIED: a ~170-file diff cannot be meaningfully read, so correctness
does not rest on reading it. The parser was taught BOTH formats first, so the
body-diff guard parses the old form at the merge-base and the new form at head --
the migration validates itself, no bypass. The proof is a field-level equivalence
harness: 168 records before and after, zero lost, zero gained, zero field
mismatches, zero rationale bodies differing. Reviewers should scrutinise the
harness; it is the actual evidence.

What measuring caught that reading would not have:

- ~500 lines sit OUTSIDE any record -- decisions.md's lifecycle schema and each
  topic file's preamble, mostly the only copy. Source files are kept and
  stripped, never deleted. They also cannot be filed per-area: topic files hold
  several areas and 4 of 23 areas span several files.
- Archive discovery was a non-recursive glob; after the split it found ZERO
  archived records, surfacing as four bogus "supersedes points to unknown key"
  errors rather than an obvious failure.
- ~32 live docs point into the corpus BY DATE, which the split dangles. Each
  stripped file now ends with a generated "Records formerly in this file" index,
  which also rescues the identical breadcrumbs in old issue comments.
- decisions.md's "In this file:" list was 97 same-file anchor bullets that the
  split makes WRONG, not merely stale. Dropped; the generated index replaces
  them with links that resolve.

The equivalence harness now runs against a checked-in FIXTURE, not the live
corpus. The earlier version migrated the real tree, which made it a one-shot:
the moment the migration landed there was nothing left to move and the tests
failed for reasons unrelated to the code. A fixture keeps them testing the
SCRIPT rather than the repo's current state.

Keys preserved verbatim, warts included: `sched` (12) and `scheduling` (1) remain
two directories for one concept. Renaming a key is not a move -- it changes
identity, breaks the equivalence proof, and invalidates MemPalace's per-key
drawers. Taxonomy normalisation is separate work.

refs #610

docs/decisions/records/testing/playwright-mcp-download-and-recovery.md

@@ -0,0 +1,17 @@
+---
+key: testing.playwright-mcp-download-and-recovery
+title: '2026-07-21 — Playwright-MCP: curl download endpoints, never open a tab or `window.open` (#542)'
+status: active
+since: '2026-07-21'
+supersedes: none
+superseded-by: none
+rule: In Playwright-MCP E2E, fetch file-download endpoints with curl — never a browser tab or `window.open` — and if browser tools stall repeatedly, `pkill -f ms-playwright-mcp` and drive a fresh session.
+signals: 'Playwright MCP · file download endpoint · window.open · browser tab stall · `ms-playwright-mcp` · paths: n/a · issues: #542'
+mechanics: '`pkill -f ms-playwright-mcp` then re-drive from a fresh MCP session.'
+---
+
+Download endpoints hand the browser a file rather than a page; the tab has nothing to snapshot and the
+tool waits on navigation that never completes. curl gives you the bytes and the status code directly.
+
+Repeated stalls are a wedged MCP browser process, not a slow app — kill it and restart rather than
+retrying the same call.