schema stamps: lazy v1 for chats + the graph family, honest refusal

Chat records gain version (CURRENT_CHAT_VERSION), stamped by a single
writeChat choke point in the store; absence reads as v1 — existing files
migrate on their next write, no mass rewrite. The graph family (four JSONL
ledgers, sources.json, render-settings.json) shares one lazy sidecar,
graph/format.json; positions stays unstamped (rebuildable) and jobs stay
unstamped (transient).

Downgrade scenario ruled honest-refusal: records stamped newer than the
running build read as absent (chats, logged) or throw (graph family) — a
read-modify-write store never rewrites what it doesn't understand.

Golden fixtures stay the unstamped legacy shape on purpose (lazy v1 is
current-valid by design); the stamped write path is pinned by
tests/schema-stamps.test.ts. Reads never write — the golden byte-identical
pin holds.

tests/fixtures/golden-data/README.md

@@ -36,3 +36,12 @@ core tool surface must appear in real data here.
 Run the app (`npm run dev`), exercise the feature against `data/`, then copy
 the produced record in verbatim. Do not hand-edit content; if a record needs
 pruning, re-run the flow instead.
+
+## Stamps
+
+These files are deliberately the UNSTAMPED legacy shape (schema-stamp wave,
+2026-10-10): lazy v1 means absence reads as current-valid, so this set pins
+the pre-stamp read path forever. Stamped writes (chat `version: 1`, the
+`graph/format.json` sidecar) are pinned by `tests/schema-stamps.test.ts`.
+New records copied in after their first post-stamp write will carry the
+stamps — that shape is equally current-valid.