Cache session discovery and stabilize graph sweep

[Yehonal]

Change

• Cache transcript facts by file offset and read only appended records on warm discovery.
• Use runtime metadata for session titles, surface and attention signals; keep fixture and background sessions ephemeral.
• Keep unchanged sweep observations out of the event log and refresh the graph projection on no-op sweeps.
• Add snapshot compaction and a read-only fixture cleanup inventory; isolate launch-option test state.
• Preserve the existing lifecycle and resumability fixes from the prior graph worktree.

Validation

• npm run verify passed.
• python3 packages/tmux-bridge/scripts/session-facts-cache-test.py passed (2 tests).
• node --test packages/tmux-bridge/test/mesh-graph.test.mjs passed (10 tests).

Rollout

Source change only. Deployment, service changes, graph backup, fixture purge, and compaction require separate operator approval and verification.