refactor(ci): split workspaces-a into two Vitest native shards

Split the slow workspaces-a CI lane into two Vitest native shards and keep release verification in parity.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

scripts/__tests__/release-verify-workflow.test.mjs

@@ -84,6 +84,15 @@ test("release verify workflow covers the same split test surface as stable PR ve
     assert.match(verifyWorkflow, new RegExp(`shard_index: ${shardIndex}[\\s\\S]*?shard_count: 5`));
   }
 
+  // workspaces-a splits with Vitest native --shard in pr.yml; release
+  // verification must keep the same two-shard coverage.
+  for (const shardIndex of [0, 1]) {
+    assert.match(
+      verifyWorkflow,
+      new RegExp(`group: general-workspaces-a[\\s\\S]*?shard_index: ${shardIndex}\\n\\s+shard_count: 2`),
+    );
+  }
+
   assert.match(verifyWorkflow, /pnpm test:run:general -- --group/);
   assert.match(verifyWorkflow, /pnpm test:run:serialized -- --shard-index/);
 });

scripts/__tests__/run-vitest-stable-shard.test.mjs

@@ -77,9 +77,37 @@ test("a route/authz suite never leaks into the general-server shards", () => {
   }
 });
 
-test("shard flags are rejected for the parallel workspace groups", () => {
-  const result = dryRun(["--mode", "general", "--group", "general-workspaces-a", "--shard-index", "0", "--shard-count", "3"]);
-  assert.notEqual(result.status, 0, "workspace groups must not accept shard flags");
+test("shard flags are rejected for the workspaces-b group", () => {
+  const result = dryRun(["--mode", "general", "--group", "general-workspaces-b", "--shard-index", "0", "--shard-count", "3"]);
+  assert.notEqual(result.status, 0, "workspaces-b must not accept shard flags");
+});
+
+test("workspaces-a shards map to Vitest native --shard slices over a stable project list", () => {
+  const shards = [0, 1].map((index) =>
+    dryRunJson([
+      "--mode", "general", "--group", "general-workspaces-a",
+      "--shard-index", String(index), "--shard-count", "2",
+    ]),
+  );
+
+  assert.deepEqual(
+    shards.map((shard) => shard.workspacesVitestShard),
+    ["1/2", "2/2"],
+    "each matrix job must pass its own --shard slice to vitest",
+  );
+  // Vitest's --shard partitions each project's file list deterministically, so
+  // an identical project list across jobs is what guarantees complete,
+  // non-overlapping coverage of the lane.
+  assert.deepEqual(shards[0].workspaceProjects, shards[1].workspaceProjects);
+  assert.ok(shards[0].workspaceProjects.length > 0, "workspaces-a must run at least one project");
+
+  const unsharded = dryRunJson(["--mode", "general", "--group", "general-workspaces-a"]);
+  assert.deepEqual(
+    unsharded.workspaceProjects,
+    shards[0].workspaceProjects,
+    "sharding must not change which projects the lane covers",
+  );
+  assert.equal(unsharded.workspacesVitestShard, null);
 });
 
 test("duration-aware partition balances skewed weights better than round-robin", () => {