fix: metrics wait for their session row instead of racing it to a 409 (GIT-73)

[nTEG-dev]

What

Metrics writes wait for their session row instead of racing it to a 409 (GIT-73).

• session_start registers its fire-and-forget session insert with registerPendingSessionRow().
• recordMetrics waits for that insert, bounded at 15 s, before writing any row that references the session. That covers session_start's own metrics and anything recorded right after it, e.g. recall.
• If the session row didn't land, or didn't within the budget, the metrics row is written with session_id: null and keeps the id in metadata.session_id. That's where session_close already looks for it (GIT-109), so the row is kept instead of lost to a 409.
• Sessions this process didn't create (resumed, or another process's) are written immediately, as before.
• session_start itself still doesn't wait: the session insert stays fire-and-forget, and only the metrics write is sequenced after it.

Why

gitmem_query_metrics.session_id references gitmem_sessions(id). session_start sent the session insert and its metrics insert at the same moment, so whichever reached the store first decided whether the metrics insert failed with 409 (FK violation).

How to test

Venue (v1.8.0 setup.sql, vujndhcumgroztfkkdus; production not touched). GIT-73 is removed from the driver's EXPECTED_FAILURES, so any 409 now fails the run. Ten flow runs each:

	origin/main 1.10.0	This branch
Runs with a 409 on gitmem_query_metrics	7 / 10 (exit 1)	0 / 10 (exit 0)
409s total	7	0

for i in $(seq 10); do VENUE_ENV=<env> node tests/e2e/blank-supabase.mjs flow --label git73-r$i --out <dir>; done

Unit tests: 1279/1279 pass (+5, metrics-session-fk.test.ts):

• the write is held until the session row lands, then keeps the FK;
• a row that didn't land → session_id: null, id in metadata.session_id;
• creation threw → treated as not landed;
• later writes reuse the settled answer;
• an unregistered session is written immediately.

No existing test encoded the old behaviour.

Checklist

• npm run test:unit passes (1279)
• New tests added
• No schema change; verified on a v1.8.0 store, 10 runs
• Docs: n/a

Breaking changes

None.

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