fix(realtime): keep spill outside the write mutex

[HaHaJeff]

Purpose

Follow up on #327 to avoid blocking realtime writes on sealed-segment spill I/O in append-only and primary-key writers.

• Serialize Seal() and PrepareCommit() with prepare_mutex_.
• Run the built-in ArrowRealtimeStore::SealForCommit() outside realtime_store_mutex_. The store already protects segment rotation internally and spills outside its own mutex, allowing writes to the next building segment during spill.
• Reacquire the write mutex to publish sealed segments and derive has_building_data_ from the current offset and sealed range, preserving data written during spill.

Tests

Local validation completed for this fix:

• Realtime core tests: 94/94 passed.
• Realtime integration tests: 77/77 passed.
• Controlled temporary concurrency validation: 4/4 cases passed; this validation was not added as a permanent test.
• git diff --check: passed.
• pre-commit was unavailable, so its checks were not run.

API and Format

No public API, storage format, or protocol changes.

Documentation

No documentation changes; this fixes the existing spill behavior.

Generative AI tooling

Generated-by: OpenAI Codex (GPT-6)

[lxy-9602]

+1