fix(context): snapshot primary thread compaction

[yanpgwang]

Summary

• persist immutable context-compaction snapshots for the primary Thread as well as child Threads
• restore the first committed primary projection on a retried PrepareTurn Activity, even if context policy or token budget changes between attempts
• verify that primary events and snapshots use the existing primary Thread ledger and database constraints
• update architecture and compatibility docs to describe compaction checkpoints as per-Thread rather than child-only

The database schema and runtime already model the primary conversation as a durable Thread. The remaining child-only guard in PrepareTurn prevented primary compaction from using the same replay-safe checkpoint mechanism.

Verification

• make verify
• make test-service
• make docs-check
• focused primary/child Activity retry tests
• focused PostgreSQL primary/child snapshot tests