fix(homegrown): close post-merge soft-hold races (TIN-2764)

[Jesssullivan]

Context

PR #118 merged before its adversarial review corrections were committed. This follow-up lands the confirmed post-merge safety fixes without rewriting the merged history.

Changes

• make Homegrown soft-hold acquisition use the same practitioner-day transaction lock and occupied-set revalidation as booking writes
• derive conflict query days from the buffer-expanded interval and iterate local calendar-day lock keys across DST transitions
• query active holds by interval overlap, and exclude only the exact hold id + datetime + duration capability
• propagate SLOT_TAKEN before payment while stripping any caller-supplied hold id from the standard checkout pipeline
• start hold TTL after lock contention and the final availability check
• add unit coverage plus a real-Postgres race where exactly one of two concurrent holds wins
• update the parity matrix to reflect Homegrown atomicity and the bridge-only advisory fallback

Review and validation

• initial adversarial review found the post-lock TTL issue; fixed with a regression test
• final independent Codex review: PASS
• pnpm check: 0 errors, 0 warnings
• pnpm lint
• pnpm test:unit: 876 passed
• pnpm test:integration: 34 passed, 5 Postgres tests skipped locally without DATABASE_URL
• pnpm build
• pnpm check:package: publint all good
• nix develop --command pnpm docs:check: strict MkDocs build passed
• git diff --check

Merge gate

The CI Integration (Postgres service) job must run the five database tests, including the new concurrent-hold case, and pass before merge.

Tracks TIN-2764.