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>

test/fixtures/exit-propagation/failing.txt

@@ -0,0 +1,2 @@
+import { test, expect } from 'bun:test';
+test('this failure must be visible', () => { expect(1).toBe(2); });

test/fixtures/exit-propagation/passing.txt

@@ -0,0 +1,2 @@
+import { test, expect } from 'bun:test';
+test('passes', () => { expect(1).toBe(1); });

test/fixtures/exit-propagation/suicide.txt

@@ -0,0 +1,8 @@
+import { test, expect } from 'bun:test';
+test('passes then arms a delayed exit', () => {
+  expect(1).toBe(1);
+  setTimeout(() => process.exit(0), 300);
+});
+test('waits long enough for the timer to fire', async () => {
+  await new Promise((r) => setTimeout(r, 1500));
+});