feat(xce-test-rc-file): task 1.1 rc-file path resolution with XDG precedence

Adds rcfile_resolve_path() (rcfile.c/rcfile.h): resolves $XDG_CONFIG_HOME/xce_test/rc
when set and non-empty, else $HOME/.config/xce_test/rc, else NULL. Standalone
libc-only module mirroring the configcompare.c pattern; wired into xce_test_src
and controller_unit_test_src, registered in controller_test.c's Unity runner.

6 unit tests in test_rcfile.c cover all XDG/HOME combinations plus a dedicated
env-var save/restore isolation test, addressing the cross-test leak risk found
during design review (controller_unit_test runs 80+ test files in one process).

Adds the xce-test-rc-file spec (requirements/design/tasks/research), including
design fixes from validation: getline()-based line reading (task 1.2, avoids
truncating long rc-file lines) and the env-isolation requirement above.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AgYRU3zDTqNGV5Jk8aiKLg

.kiro/specs/xce-test-rc-file/spec.json

@@ -0,0 +1,22 @@
+{
+  "feature_name": "xce-test-rc-file",
+  "created_at": "2026-08-26T08:49:11Z",
+  "updated_at": "2026-08-26T09:18:22Z",
+  "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
+}