test(xce-harness-component): verify build isolation; narrow requirement 2.2

Validation task 4.3 found a real, deterministic conflict with the
original requirement 2.2 wording, not a flake: /xce-build controller
alone's default asan profile transitively rebuilds harness/xce_test,
because Meson's test-runner rebuilds every registered test's build-
order dependencies project-wide before filtering by --suite, and
harness_unit_test (task 2.1) depends on it. Confirmed via build-log
step-count analysis (explicit compile: 209 steps, clean; subsequent
meson test: 844 steps, resurrects it) and independently reproduced
twice, with an exact step-count match both times.

User decision (2026-09-14): accept as a documented Meson limitation
rather than auditing every component's test-target list (controller's
own suite already relies on this same mechanism for 4 of its own test
binaries) or weakening harness_unit_test's build-order guarantee
(verified load-bearing in task 4.2). Requirement 2.2 narrowed to what
the build system directly controls: its explicit compile-target list
never names xce_test. requirements.md/design.md/tasks.md updated to
match and cross-reference each other; logged as bug-1806 (accepted,
not fixed) for the record.

Also commits the xce-harness-component spec's remaining docs
(requirements.md, brief.md, research.md, spec.json) and the roadmap.md
promotion entry from discovery, none of which had been committed yet.

Task 4.3 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/spec.json

@@ -0,0 +1,22 @@
+{
+  "feature_name": "xce-harness-component",
+  "created_at": "2026-09-14T02:46:14Z",
+  "updated_at": "2026-09-14T03:35:00Z",
+  "language": "en",
+  "phase": "tasks-generated",
+  "approvals": {
+    "requirements": {
+      "generated": true,
+      "approved": true
+    },
+    "design": {
+      "generated": true,
+      "approved": true
+    },
+    "tasks": {
+      "generated": true,
+      "approved": true
+    }
+  },
+  "ready_for_implementation": true
+}