fix(storage): stabilize content verification scan race

[TaprootFreak]

Summary

• recheck only content-report keys that appear missing on the allegedly absent target after the non-atomic listings
• rebuild the full metadata, size, and needs-hash classification from the rechecked target metadata
• apply the same stabilization to report, hash fixpoint, and final gate paths without downloads, writes, overwrites, deletes, or an upload freeze
• reject large one-sided-empty inventories before per-key requests while allowing the bounded first-object race

Verification

• 59 targeted content-verification tests
• TypeScript type-check
• ESLint
• Prettier check
• git diff check

[TaprootFreak]

Review completed in 1 PR pass with 0 findings. The pre-PR review also completed in 1 pass. The change preserves full metadata, size, and needs-hash classification after target rechecks, applies the same logic to report, hash-fixpoint, and final-gate paths, and keeps large one-sided-empty inventories fail-closed before per-key requests. Local targeted tests, type-check, lint, formatting, and diff validation are green; CI is still running.