Keep the Windows E2E jobs sequential

Running the Windows suite in parallel cut it from ~21 min to ~5.5 min, but a
`--parallel 1` control run on main was green while every parallel run failed on a
different test — six distinct tests across three runs, never the same one twice.
Parallelism exposes latent timing races rather than breaking anything
deterministically, and lowering Bun to 2 did not help either.

Revert the workflow to sequential and keep only the changes that are independent
of parallelism. Enabling it needs those races fixed first; one root cause is the
hardcoded 5s IPC-credential timeout in the bridge, recorded in #353.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HUzrgoGWLirnc3zyeVZLz5

Author: claude

.github/workflows/e2e-windows.yml

@@ -30,15 +30,7 @@ jobs:
         run: pnpm run build
 
       - name: E2E tests
-        # windows-latest runners have 4 vCPUs, so run 4 tests at a time (same as
-        # macOS). Process spawning is far more expensive on Windows than on Linux
-        # and the suite is spawn-bound, so it was previously run sequentially and
-        # took ~21 min — by far the slowest job in CI.
-        env:
-          # Individual tests slow down under parallel contention; the watchdog only
-          # needs to catch genuine hangs, so give it headroom over the 180s default.
-          E2E_PER_TEST_TIMEOUT: 300
-        run: ./test/e2e/run.sh --no-build --parallel 4
+        run: ./test/e2e/run.sh --no-build --parallel 1
 
   e2e-windows-bun:
     name: E2E tests (Windows, Bun)
@@ -66,10 +58,4 @@ jobs:
         run: pnpm run build
 
       - name: E2E tests
-        # Bun on Windows is less tolerant of contention than Node: at --parallel 4
-        # it intermittently failed with bridge startup errors on tests that passed
-        # in other runs, so this job uses 2. Still roughly half the previous
-        # sequential runtime. See the note in e2e-windows-node above.
-        env:
-          E2E_PER_TEST_TIMEOUT: 300
-        run: ./test/e2e/run.sh --no-build --runtime bun --parallel 2
+        run: ./test/e2e/run.sh --no-build --runtime bun --parallel 1