perf(sessions): skip codex rescans for unchanged rollouts

[ScriptedAlchemy]

Refs #2658, #2521.

Cause

Every history pass, each unchanged resumed Codex rollout:

• rebuilt its prior context by re-reading the rollout up to its cursor (CodexContextState::scan_prior), even when the scan had no frame. The prior-context cache holds 512 rollouts, so past that every pass re-read every rollout;
• built a ProjectRootMatcher (a git spawn) for its scope, even when no frame consulted it.

Change

• Rebuild prior context only when the scan has a frame.
• Resolve the scope matcher lazily, once, on the first frame that consults it.

Evidence

• Test an_unchanged_resumed_rollout_does_not_rebuild_its_prior_context (sessions lib):

  • after the prior-context cache is evicted, an unchanged pass does 0 rescans and persists 0 frames;
  • an appended record leads to 1 rescan and 1 persisted frame;
  • it fails without the guard (1 rescan instead of 0).

• Isolated hotpath daemon on the 8× store (about 60.6k messages; 120 streamed messages per window, 183 ingested):

  	Before	After
  Raw bytes per message	32.4 MB / 19.3 MB	5.05 MB / 3.11 MB
  matcher_new	17,693 calls (186 s)	0
  scan_prior	17,600 calls	0

• For comparison, the base store (about 7.6k messages) reads 2.48 MB / 2.05 MB per message.

• Suites:

  Suite	Passed
  sessions lib	527
  application lib	483
  application_suite	66
  session_suite	433
  mcp_suite	614

• clippy -D warnings and fmt are clean.

[changeset-bot]

⚠️ No Changeset found

Latest commit: de1412c

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR