feat(core): add durable compaction barrier

[kitlangton]

Summary

• generalize session_input into a typed durable inbox for prompt and compaction entries
• admit and coalesce one manual compaction barrier instead of rejecting active sessions
• block every unpromoted steer/queue behind the barrier, finish required active continuation, compact at the would-be-idle boundary, then release the backlog
• project one stable separator through ◇ Compaction queued → animated Compaction → ✓ Compaction, with streamed summary prose beneath
• preserve automatic compaction and build on feat(core): finalize session event lifecycle #35272 for live compaction deltas
• migrate nonempty prompt inbox rows into the tagged schema and regenerate current/legacy clients

This PR is stacked on #35272 (event-fragments). After that merges, it can retarget v2.

Demo

compaction-ui-corrected.mp4

Verification

• full repository pre-push typecheck: 31/31 packages
• Core Session suite: 255 tests
• Core migration suite: 17 tests
• TUI suite: 207 passed, 1 skipped
• focused Schema, Client, runner barrier, admission conflict, migration, projection, and row-order tests
• Terminal Control with a delayed local provider, showing queued barrier, separator spinner, streamed summary, completion check mark, and backlog release