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/plan-phase3-results-ux.md

@@ -1,6 +1,7 @@
 # cc-ci Phase 3 — Beautiful YunoHost-style results (Autonomous Build Plan)
 
-**Status:** QUEUED — starts only after Phase 2 (`plan-phase2-recipe-tests.md`) reaches `## DONE`.
+**Status:** QUEUED — starts after Phase 2 (`plan-phase2-recipe-tests.md`) and Phase 2b
+(`plan-phase2b-test-performance.md`) reach `## DONE`.
 **Transition:** **manual** (operator kicks it off; check in / test between phases).
 **Builds on:** Phase 1 (`plan.md` — dashboard `dashboard/`, the `!testme` bridge's PR comment,
 the runner, Playwright in the harness) and Phase 2 (the rich per-recipe test taxonomy → meaningful