fix(test): remove all 8 delayed process.exit teardown bombs — the tier-1 gate can finally fail

bun test runs every file in ONE process, so a 500ms setTimeout(process.exit(0))
armed in afterAll fired mid-way through a LATER file and killed the entire
suite with exit 0 and no summary — only ~16 of 434 files ran, and every
downstream failure was invisible (observed live throughout this wave's
enumeration). Changes, all guarded by fault injection:

- Replace every delayed-exit teardown with a time-boxed close of the file's
  own browser (8 files across browse/ and design/); stub the daemon
  /shutdown timer instead of letting its unconditional process.exit tear
  the runner down.
- test/no-suicide-exit.test.ts: static tripwire — no *.test.ts may schedule
  a delayed process.exit again.
- test/exit-propagation.test.ts + fixtures: fault injection with REAL bun
  output proves the truncation shape (exit 0, no summary) and that
  scripts/test-free-shards.ts now detects it: a shard exiting 0 WITHOUT
  bun's final summary line is treated as FAILED (exit code alone is not
  evidence of completion).
- handoff: the three headed-mode integration tests are darwin-skipped with
  a pointer to the known macOS headed-launch breakage (#2242/#2554); they
  keep running on Linux CI. Un-skip in the browse-daemon wave.
- feedback-roundtrip: repair the handler call sites unmasked by the fix —
  handlers take (command, args, session, bm); passing the manager where a
  session belongs broke all six tests.
- user-slug-fallback: HOME isolation makes endpoint_hash deterministic.

Fixes #2421, #2435.

Contributed by @sneakygriff (PR #2172) with repairs from @time-attack
(PR #2230 feedback-roundtrip hunks); supersedes PR #2252 by @whd4 (same
defect, credited).

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

browse/test/batch.test.ts

@@ -42,9 +42,14 @@ beforeAll(async () => {
   // The test needs to start a server. Let's use the existing server infrastructure.
 });
 
-afterAll(() => {
+afterAll(async () => {
   try { testServer.server.stop(); } catch {}
-  setTimeout(() => process.exit(0), 500);
+  // Close only this file's own browser — never process.exit(): bun test runs
+  // all files in one process, so a delayed exit kills the whole suite
+  // (see test/no-suicide-exit.test.ts). close() can hang when the browser
+  // already died, and its internal 5s timeout ties bun's 5s hook timeout —
+  // so race it at 3s and abandon; the child is reaped at process exit.
+  try { await Promise.race([bm?.close(), new Promise((resolve) => setTimeout(resolve, 3000))]); } catch {}
 });
 
 // We need a running browse server for HTTP tests.