test: introduce planner-backed test runner, stabilize local builds (#54650)

* test: stabilize ci and local vitest workers

* test: introduce planner-backed test runner

* test: address planner review follow-ups

* test: derive planner budgets from host capabilities

* test: restore planner filter helper import

* test: align planner explain output with execution

* test: keep low profile as serial alias

* test: restrict explicit planner file targets

* test: clean planner exits and pnpm launch

* test: tighten wrapper flag validation

* ci: gate heavy fanout on check

* test: key shard assignments by unit identity

* ci(bun): shard vitest lanes further

* test: restore ci overlap and stabilize planner tests

* test: relax planner output worker assertions

* test: reset plugin runtime state in optional tools suite

* ci: split macos node and swift jobs

* test: honor no-isolate top-level concurrency budgets

* ci: fix macos swift format lint

* test: cap max-profile top-level concurrency

* ci: shard macos node checks

* ci: use four macos node shards

* test: normalize explain targets before classification

vitest.extensions.config.ts

@@ -14,7 +14,6 @@ export function createExtensionsVitestConfig(
   return createScopedVitestConfig(loadIncludePatternsFromEnv(env) ?? ["extensions/**/*.test.ts"], {
     dir: "extensions",
     env,
-    pool: "forks",
     passWithNoTests: true,
     // Channel implementations live under extensions/ but are tested by
     // vitest.channels.config.ts (pnpm test:channels) which provides