test(FN-3607): harden test workflow verification and isolation checks

- Expand test-changed coverage and shard selection assertions for CI workflows
- Improve vitest worker temp-directory utilities and related core/CLI tests
- Refine test isolation guardrails and runtime ignore handling for live .fusion noise
- Update contributing guidance and root test script usage for the verified workflow

Fusion-Task-Id: FN-3607

packages/core/src/__test-utils__/vitest-workers.ts

@@ -4,6 +4,10 @@ interface ComputeMaxWorkersOptions {
   defaultCap?: number;
 }
 
+function computeDefaultCap(cpuCap: number): number {
+  return Math.max(2, Math.min(6, Math.ceil(cpuCap / 2)));
+}
+
 function parsePositiveInt(value: string | undefined): number | undefined {
   const parsed = Number.parseInt(value ?? "", 10);
   if (!Number.isFinite(parsed) || parsed <= 0) return undefined;
@@ -17,13 +21,12 @@ function parsePositiveInt(value: string | undefined): number | undefined {
 //   2. FUSION_TEST_TOTAL_WORKERS — global budget across the workspace, divided
 //      by FUSION_TEST_CONCURRENCY (default 1). Lets `pnpm -r` runs cap total
 //      fan-out instead of multiplying per package.
-//   3. defaultCap — small ceiling (2 by default) so a single package run on a
-//      high-core machine stays gentle.
+//   3. defaultCap — CPU-aware default for local single-package runs so modern
+//      machines can use more parallelism without runaway fan-out.
 // All paths clamp to (cpus - 1) so we never oversubscribe.
 export function computeMaxWorkers(options: ComputeMaxWorkersOptions = {}): number {
-  const { defaultCap = 2 } = options;
-
   const cpuCap = Math.max(1, cpus().length - 1);
+  const { defaultCap = computeDefaultCap(cpuCap) } = options;
 
   const explicit = parsePositiveInt(process.env.VITEST_MAX_WORKERS);
   const totalBudget = parsePositiveInt(process.env.FUSION_TEST_TOTAL_WORKERS);

packages/core/src/__tests__/vitest-workers.test.ts

@@ -43,7 +43,7 @@ describe("computeMaxWorkers", () => {
     expect(process.env.VITEST_MAX_WORKERS).toBe("3");
   });
 
-  it("ignores invalid env values and falls back to default cap", () => {
+  it("ignores invalid env values and falls back to provided default cap", () => {
     process.env.VITEST_MAX_WORKERS = "abc";
     process.env.FUSION_TEST_TOTAL_WORKERS = "0";
     process.env.FUSION_TEST_CONCURRENCY = "-1";
@@ -53,4 +53,17 @@ describe("computeMaxWorkers", () => {
     expect(workers).toBe(2);
     expect(process.env.VITEST_MAX_WORKERS).toBe("2");
   });
+
+  it("uses a CPU-aware default cap when no overrides are provided", () => {
+    delete process.env.VITEST_MAX_WORKERS;
+    delete process.env.FUSION_TEST_TOTAL_WORKERS;
+    delete process.env.FUSION_TEST_CONCURRENCY;
+
+    const cpuCap = Math.max(1, cpus().length - 1);
+    const expected = Math.max(2, Math.min(6, Math.ceil(cpuCap / 2)));
+    const workers = computeMaxWorkers();
+
+    expect(workers).toBe(expected);
+    expect(process.env.VITEST_MAX_WORKERS).toBe(String(expected));
+  });
 });