fix: repoint stale controller/xce_test path references to harness/xce_test

Follow-up to the xce-harness-component spec, which relocated xce_test
from src/controller/testsuite/ into a standalone src/harness/
component (commits cf55d691..cf380750). These four files were outside
that spec's own file boundary (owned by other specs / general tooling)
so were deliberately left alone at the time, but their hardcoded
default path and error-message advice were now stale and would have
misled anyone who hit them:

- .claude/skills/xce-build/SKILL.md: added `harness` to both component
  lists (frontmatter description + args section), and replaced the
  "controller always builds xce_test too" claim with an accurate
  description of the harness component, its own build command, and
  the accepted Meson test-runner quirk (bug-1806) that can still
  transitively rebuild xce_test as a side effect of another
  component's default-profile test run.
- src/controller/testsuite/test_rcfile_integration.sh (owned by the
  xce-test-rc-file spec): XCE_TEST_BIN default and failure message.
- src/controller/testsuite/clean_failover_preservation.sh (owned by
  ha-term-safety task 5.2): same.
- src/tools/test-commit-durability.sh: same, plus its header comment.

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

src/tools/test-commit-durability.sh

@@ -80,7 +80,7 @@
 #       reports RED (no primary fdatasync precedes the ack). After the fix it must
 #       go GREEN. See run_deterministic() below.
 #
-# Requires a current build: run /xce-build controller first.
+# Requires a current build: run /xce-build controller and /xce-build harness first.
 set -u
 
 # ── mode selection ────────────────────────────────────────────────────────────
@@ -94,7 +94,7 @@ fi
 
 WORKTREE=$(cd "$(dirname "$0")/../.." && pwd)
 BINARY="$WORKTREE/src/xce.build/controller/xce_controller"
-XCE_TEST="$WORKTREE/src/xce.build/controller/xce_test"
+XCE_TEST="$WORKTREE/src/xce.build/harness/xce_test"
 DEPLOY_DIR="${1:-/tmp/xce-ha-deploy}"
 ROUNDS="${2:-8}"
 PEERS="localhost:9991,localhost:9992,localhost:9993"