fix(storage): await IndexedDB transaction completion

[LotemAm]

IndexedDB request success can be followed by transaction rollback, causing callers to acknowledge a cache write that never committed. Retain the transaction and return the request result only from its completion event; preserve request and transaction errors, with AbortError only when no error is available.

Limit the existing closed-connection retry to transaction creation, so an issued operation is never replayed. Add real fake-indexeddb coverage for success followed by abort, completion ordering, readonly/missing values, request and subsequent-transaction errors, and synchronous operation errors.

Validation (Node 24.16.0 / Yarn 4.17.0):

• Regression RED confirmed: 2 expected failures, 5 existing tests passed.
• Focused IndexedDB/cache/handle/enrollment suites: 4 files / 21 tests passed.
• Full unit suite: 52 files / 432 tests passed.
• yarn typecheck, yarn lint, and git diff --check passed.

Implements Plan 002; depends on merged #27. Per the requested lifecycle, merge after green CI and clear actual review state; no Codex reviewer is configured.