fix(compression): make continuation rotation atomic

[trac3r00]

[Bob]

Summary

• close a compression parent and create its continuation child in one BEGIN IMMEDIATE transaction
• publish the new in-memory session id only after the atomic DB rotation succeeds
• add a deterministic WAL race regression proving a concurrent reader/pruner can never observe an ended parent without its child

Root cause

Legacy rotation called end_session() and create_session() as separate transactions. Auto-prune could select and delete the newly ended parent between those writes, so the child insert lost its lineage target and the live agent could continue with a session id that was never persisted.

Verification

• uv run pytest -q tests/agent/test_compression_rotation_state.py tests/agent/test_compression_concurrent_fork.py tests/test_hermes_state.py — 435 passed
• race regression repeated 40/40 successfully
• mutation probe against the old split sequence exposed parent_ended=true, child_visible=false, confirming the test distinguishes the regression
• uv run ruff check agent/conversation_compression.py hermes_state.py tests/agent/test_compression_rotation_state.py — clean
• independent review — PASS, no blockers

[alt-glitch]

This was generated by AI during triage.

Duplicate of #64373. Both patches atomically complete compression parent-to-child rotation; #64373 is the existing broader implementation, including durable lease validation and regression coverage.

[trac3r00]

Confirmed. #64373 predates this PR and covers the same atomic parent-to-child rotation while also adding durable lease validation and broader concurrency regression coverage. Closing this duplicate in favor of #64373; credit to @yingliang-zhang for the earlier, more complete implementation.

[bob]