Add Phase-2b plan: test performance (measure, attribute, improve empirically)

Phase 2b (after Phase 2, before Phase 3): instrument per-phase timings, baseline a
representative recipe set (cold vs warm), attribute where time goes (Pareto), then try
improvements as controlled before/after experiments and keep measured winners — image
pull cache/pre-pull, readiness-wait tuning, dedup deploy cycles, warm/shared infra
(isolation-proven), runner caching, concurrency sizing, vCPU. Speed never weakens tests
or isolation (Adversary re-measures + re-verifies). Phase 3 now follows 2b. Linked in README.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cc-ci-plan/README.md

@@ -16,7 +16,8 @@ autonomous Claude loops (a Builder and an adversarial Reviewer) running over day
 |---|---|
 | `plan.md` | The Phase-1 plan (build the CI server). Agents treat it as their single source of truth. |
 | `plan-phase2-recipe-tests.md` | **Phase 2** (after Phase-1 `## DONE`): author comprehensive per-recipe tests — port every recipe-maintainer test + ≥2 recipe-specific tests per app. |
-| `plan-phase3-results-ux.md` | **Phase 3** (after Phase-2 `## DONE`): beautiful YunoHost-style results — per-run **level**, image-forward PR comment (badge + summary card + app screenshot), polished dashboard. |
+| `plan-phase2b-test-performance.md` | **Phase 2b** (after Phase 2, before Phase 3): empirically measure where test time goes and reduce it (image cache, readiness tuning, dedup deploys, warm infra, concurrency) — no weakened tests. |
+| `plan-phase3-results-ux.md` | **Phase 3** (after Phase 2b): beautiful YunoHost-style results — per-run **level**, image-forward PR comment (badge + summary card + app screenshot), polished dashboard. |
 | `IDEAS.md` | Deferred/future ideas, parked out of current scope. |
 | `brief.md` | The original one-page brief (context only; `plan.md` supersedes it). |
 | `kickoff.md` | Launch & supervision guide. |