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/concurrency/schedule-item-child-identity.md

@@ -0,0 +1,33 @@
+---
+key: concurrency.schedule-item-child-identity
+title: '2026-07-11 — Stable child identity for schedule-item replace (#259, split from #252/#253)'
+status: active
+since: '2026-07-11'
+supersedes: none
+superseded-by: none
+rule: '`PUT /api/schedules/{id}/items` reconciles by an optional round-tripped child `Id` (null/absent/0 ⇒ new item), never by array position, so fill-group/shuffle state follows the logical item across reorders; an unknown or duplicate id is rejected 422 (checked after the §7a `CheckVersion`, so 412 precedes 422).'
+signals: 'stable child identity, schedule-item replace, id-based reconcile · paths: `api-conventions.md` §7c · issues: #259, #252, #253, #197'
+mechanics: '`docs/api-conventions.md` §7c'
+---
+
+`PUT /api/schedules/{id}/items` now reconciles by an optional round-tripped child id, not by array
+position, so an item's persisted fill-group/shuffle state (`PlayoutScheduleItemFillGroupIndex`, FK
+`OnDelete(Cascade)`) follows the logical item across reorders/inserts instead of being inherited by
+whatever previously held its new slot. Contract + rules in **api-conventions §7c**. Key decisions:
+
+- **`ScheduleItemRequest.Id` (`int?`)**: null/absent/`0` ⇒ new item (controller normalizes `0`→null so the
+  handler is two-state). Any id present ⇒ id-based reconcile; a fully id-less payload keeps the verbatim
+  positional fallback (legacy; retires with the §7a Phase-2 `If-Match`→428 flip).
+- **Unknown or duplicate id ⇒ 422, nothing persisted**; the guards live in the handler **after** §7a
+  `CheckVersion`, so **412 precedes 422** — a client that is both version-stale and id-stale gets the reload
+  signal, not a payload-bug signal. Rationale for reject-not-insert on an unknown id: under Phase-1
+  force-write a stale id is a live lost-update signal, so silently inserting-as-new would duplicate the item
+  and return a different id than the client sent (the exact class §7a exists to surface). This is also the
+  correct #197 posture — never honor an unrecognized identifier.
+- **Scope = schedule items only.** Blocks/templates/deco-templates/playlists stay positional: their children
+  are stateless config rows (no FK'd state to misattribute; #3/#4 have no GET child id). Child ids are added
+  only where a child row anchors server-side state; the contract can be retrofitted per-endpoint later
+  (field stays optional) — so this is not #197 ossification pressure.
+- **TPT subtype change at a matched id** stays delete+insert (EF can't retype in place); state resets and a
+  new id is returned, so the SPA must re-seed item state from the PUT response (a stale id on a second save
+  now 422s).