perf(core): batch semantic vector prepare transactions

[phernandez]

Summary

• plan every entity in a semantic vector prepare window before opening the shared write scope
• apply all mutation plans through one session and one commit per window
• preserve skip-only results and surface the same shared write failure to every mutation-dependent entity

Related issue

Part of #1110

Testing

• just fast-check
• 57 focused semantic vector synchronization tests
• just fast-test (74 passed, 1 skipped)
• just package-check
• just doctor

Risk

Low to moderate. The embedding flush size and batching defaults are unchanged; this only consolidates database writes for an already-planned prepare window. Regression tests cover SQLite and Postgres transaction counts, skip-only windows, and shared write failures.