fix(browse): isProcessAlive uses signal-0 on every platform — no more tasklist probe (#1952)

Replace the Windows tasklist shell-out in isProcessAlive with
process.kill(pid, 0), unifying all platforms on the POSIX idiom. Node maps
signal-0 to an OpenProcess existence check on Windows — and the Windows
daemon runs under Node (dist/server-node.mjs + bun-polyfill, the documented
oven-sh/bun#4253 fallback) — so the probe is portable.

Why the shell-out had to go, beyond the cosmetic conhost flash the watchdog
blinked into the foreground every 60s (#1952): a Bun.spawnSync that hits
its timeout still RETURNS with partial stdout, so the `.includes()` PID
match answered "dead" for LIVE processes under load — the false-negative
half of the #2414/#2295 leak chain. Signal 0 spawns nothing, cannot time
out, and is ~5 orders of magnitude faster (measurements in #2414). EPERM
still reports alive (process exists, we just can't signal it).

Layered on the post-#2414-absorb shape: test 3 in
process-liveness-windows.test.ts now asserts the probe is subprocess-free
on ANY platform (win32 exemption dropped), test 4's static tripwire loses
its error-handling.ts exemption (a `tasklist … PID eq` existence probe
anywhere in src/ now fails CI), and windows-spawn-hide.test.ts drops its
tasklist-in-error-handling needle (nothing spawns, which is stronger than
hiding the window).

Tests: process-liveness-windows + windows-spawn-hide + error-handling —
17 pass, 0 fail.

Fixes #1952.

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

browse/test/windows-spawn-hide.test.ts

@@ -39,8 +39,9 @@ describe('windowsHide on Windows-reachable spawns (#1835)', () => {
   });
 
   test('Windows-only process probes pass windowsHide', () => {
-    // tasklist in isProcessAlive — runs in polling loops.
-    expectHideNearEvery(SRC('error-handling.ts'), "'tasklist'");
+    // isProcessAlive no longer spawns anything (signal-0 on every platform,
+    // #1952) — process-liveness-windows.test.ts pins that it stays
+    // subprocess-free, which is stronger than hiding a window.
     // powershell DPAPI + tasklist in cookie import.
     const cookie = SRC('cookie-import-browser.ts');
     expectHideNearEvery(cookie, "'powershell'");