fix(test-runner): flaky-retry vetoes on ANY unattributable failure evidence

The gate equated 'some failure attributed' with 'all failures attributed': a
shard with one attributed failure plus a headerless failure, an unhandled
error between tests, or a truncated run (no terminal summary) qualified for
retry — re-running only failingFiles and masking the rest as FLAKY-PASS,
re-opening the silent-truncation hole the strict classifier closes.
FreeShardOutcome now carries unattributedFailures; nonzero vetoes the retry.
Pins: mixed shard, truncated-with-attributed shard, empty-shard field values.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/test-free-shards-sandbox-knobs.test.ts

@@ -94,6 +94,23 @@ describe('test-free-shards: FreeShardOutcome.failingFiles (flaky-retry feed)', (
     const outcome = await runFreeShard(['planted'], 1, 1, { commandFor, quiet: true, log: () => {} });
     expect(outcome.status).toBe('failed');
     expect(outcome.failingFiles).toEqual(['test/planted.test.ts']);
+    expect(outcome.unattributedFailures).toBe(0); // fully attributed — retry-eligible
+  });
+
+  test('a MIXED shard (one attributed + one headerless failure) is flagged unattributable — retry must not mask the headerless one', async () => {
+    // The retry gate must not equate "some failure attributed" with "all
+    // failures attributed": re-running only test/planted.test.ts and passing
+    // would report the suite green over the headerless failure.
+    const commandFor = commandPrinting([
+      failLine('headerless failure before any file chunk'),
+      'test/planted.test.ts:',
+      failLine('planted failure'),
+      summary(2, 1),
+    ]);
+    const outcome = await runFreeShard(['mixed'], 1, 1, { commandFor, quiet: true, log: () => {} });
+    expect(outcome.status).toBe('failed');
+    expect(outcome.failingFiles).toEqual(['test/planted.test.ts']);
+    expect(outcome.unattributedFailures).toBeGreaterThan(0);
   });
 
   test('failures across two files attribute both; a crashed worker file joins the set deduped', async () => {
@@ -128,5 +145,18 @@ describe('test-free-shards: FreeShardOutcome.failingFiles (flaky-retry feed)', (
     const outcome = await runFreeShard(['truncated'], 1, 1, { commandFor, quiet: true, log: () => {} });
     expect(outcome.status).toBe('failed');
     expect(outcome.failingFiles).toEqual([]);
+    expect(outcome.unattributedFailures).toBeGreaterThan(0); // truncation counts as unattributable evidence
+  });
+
+  test('a truncated run WITH an attributed failure is still unattributable — tests after the cut never ran', async () => {
+    const commandFor = commandPrinting([
+      'test/planted.test.ts:',
+      failLine('planted failure'),
+      // no terminal summary: the child died mid-suite
+    ]);
+    const outcome = await runFreeShard(['planted', 'neverran'], 1, 1, { commandFor, quiet: true, log: () => {} });
+    expect(outcome.status).toBe('failed');
+    expect(outcome.failingFiles).toEqual(['test/planted.test.ts']);
+    expect(outcome.unattributedFailures).toBeGreaterThan(0);
   });
 });

test/test-free-shards.test.ts

@@ -260,6 +260,10 @@ describe('test-free-shards: strict shard execution', () => {
       log: (l) => lines.push(l),
     });
     expect(outcome.status).toBe('passed');
+    // Mutation-caught gap: bun strips types at runtime, so a missing
+    // failingFiles here feeds undefined into the flaky-retry flatMap.
+    expect(outcome.failingFiles).toEqual([]);
+    expect(outcome.unattributedFailures).toBe(0);
     expect(lines.some((l) => /^\[test:free\] shard 7\/20: 0 files, 0s, pass$/.test(l))).toBe(true);
   });