[codex] Await FileVersion reactions for File stream writes

[rita-aga]

What changed

• Make native TemperFS File $value writes await inline StreamUpdated reactions before returning.
• Extend the File $value fast-path regression fixture with the production File -> FileVersion -> RecordVersion cascade.
• Assert direct and OData $value writes leave last_version_id populated and the newest FileVersion readable before callers proceed.

Why

A live Katagami curation synthesize session failed with parse prepared context artifact: EOF while parsing a value at line 1 column 0. Datadog showed the prepared context File.StreamUpdated Created -> Ready event and the provider caller read happening before File.RecordVersion Ready -> Ready completed. The File entity could look Ready while $value readers still observed no readable version.

The native File stream write path wrote the blob and dispatched StreamUpdated, but explicitly used await_reactions: false. That made $value PUT completion weaker than the platform contract needed by prepared context handoffs.

Validation

• cargo fmt --check
• cargo test -p temper-server --test file_value_fast_path -- --nocapture