fix(test-runner): duration-packed walls keep the per-file floor — predictions don't transfer across machines

The committed duration seed is recorded on fast CI; a syscall-supervised
sandbox replays the same files 2-4x slower. Observed post-merge: a 253-file
shard predicted ~242s was wall-killed at its predicted-x3 725s wall while
genuinely progressing (the old count heuristic guaranteed 1265s). Packed
walls may be looser than the count floor, never tighter.

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

test/test-free-shards.test.ts

@@ -654,4 +654,15 @@ describe('test-free-shards: duration-aware packing (full-suite LPT)', () => {
     // Tiny prediction: base still floors it.
     expect(wallTimeoutForPackedShard(1_000)).toBe(WALL_BASE_MS);
   });
+
+  test('packed walls never undercut the per-file floor (predictions do not transfer across machines)', () => {
+    // The duration seed is recorded on fast CI; a syscall-supervised sandbox
+    // replays the same files 2-4x slower. A 253-file shard predicted at ~242s
+    // got wall-killed at predicted×3 = 725s while genuinely progressing —
+    // the count-based floor (253 × 5s = 1265s) the runner always guaranteed
+    // must survive duration packing. Looser is allowed, tighter is not.
+    expect(wallTimeoutForPackedShard(242_000, WALL_BASE_MS, 253)).toBe(253 * 5_000);
+    // When the prediction is the larger bound, it still wins.
+    expect(wallTimeoutForPackedShard(600_000, WALL_BASE_MS, 10)).toBe(1_800_000);
+  });
 });