test(xce-harness-component): verify harness_unit_test's build-order dependency

Validation-only task, no source changes. Proved the depends: [xce_test]
edge on harness_unit_test's meson test() registration is real, not
decorative, via two experiments: (A) renaming xce_test aside and
running via meson test -- meson auto-relinked it before testing,
confirming the build-graph edge genuinely exists (verified against
meson-info/intro-tests.json); (B) renaming it aside again and invoking
harness_unit_test directly, bypassing meson's dependency-aware rebuild
-- test_test_cli.c (25) and test_xcetest_process_entry.c (4) failed
loudly with clear exec-failure messages (132 Tests 29 Failures), never
silently passing. Binary restored and checksum-verified after each
experiment; final suite confirmed green (4/20/5/12/132, 0 failures).

Independently reproduced by review with a fresh temp binary name,
including catching an ASAN_OPTIONS environment-inheritance quirk in
raw (non-meson) invocation before matching the exact result.

Task 4.2 of xce-harness-component.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLSNJhNoULvVcUhXEFafQC

.kiro/specs/xce-harness-component/tasks.md

@@ -80,7 +80,7 @@
   - Observable: no behavioral difference is found; every previously-passing scenario in that coverage still passes unchanged
   - _Requirements: 4.1, 4.2, 4.3_
   - _Depends: 3.1_
-- [ ] 4.2 Verify the harness_unit_test build-order dependency actually holds
+- [x] 4.2 Verify the harness_unit_test build-order dependency actually holds
   - Deliberately remove the built `xce_test` binary, run `harness_unit_test`, and confirm `test_test_cli.c`/`test_xcetest_process_entry.c` fail loudly rather than silently pass; rebuild normally and confirm the suite passes
   - Observable: the deliberate-removal run reports a clear failure from the two pty-spawning tests; the normal run reports the full suite green
   - _Requirements: 3.1, 3.2_