feat(build): no fork in multi-threaded XCE processes, enforced by check-no-fork

Design constraint (tech.md "Process Creation"): the controller, scheduler,
xcecd and anything that starts libxce threads never call fork, vfork,
forkpty, daemon, system, popen or uv_spawn; children start with posix_spawn()
or a systemd unit. A process proven single-threaded may fork, marked
/* xce-fork-ok: <why> */. Tests, harness, autoscaler (not finalized; bug-1969)
and the Go frontend are out of scope.

- scripts/check-no-fork.py scans src/{controller,scheduler,lib,worker}
  (comment/string/#if 0 aware); runs as meson test "no-fork rule" in the
  lib/controller/scheduler/worker suites and in the pre-commit hook.
- worker: drop xcecd's unused fork-based worker start paths
  (process_manager_start_process, process_manager_start_worker; xcecd
  starts xcecd_worker over D-Bus) and a dead #if 0 daemonize fork.
- xcecd_worker's fork/uv_spawn/system calls are marked: its call graph
  from main(), callbacks included, reaches no pthread_create or libuv
  thread-pool call.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/tools/xce-build.sh

@@ -333,7 +333,7 @@ case "$COMPONENT" in
         meson compile -C "$BUILD_DIR" "${MESON_COMPILE_JOB_OPTS[@]}" xcelib_test
         if [ "$RUN_TESTS" = 1 ]; then
             echo "=== Running lib tests ==="
-            meson test -C "$BUILD_DIR" "${MESON_TEST_JOB_OPTS[@]}" "libxce test" --print-errorlogs
+            meson test -C "$BUILD_DIR" "${MESON_TEST_JOB_OPTS[@]}" "libxce test" "no-fork rule" --print-errorlogs
         fi
         ;;
     controller)