feat: persist.c WAL writer with chained CRC32 and dedicated fsync pthread (P0-1)

- Expose persist_t struct concretely in persist.h (opaque pattern deferred)
- persist_set_workdir() derives wal/snap paths from workdir+name
- persist_init() creates WAL/snap directories, rejects NFS mounts
- persist_wal_async() writes WAL entries with chained CRC32 via zlib
- Dedicated fsync pthread fires every wal_sync_ms milliseconds
- persist_flush() forces fdatasync for tests and clean shutdown
- WAL_CHECKPOINT type rejected with -EINVAL (never written via async path)
- Stubs: persist_recover, persist_wal_sync_append, persist_checkpoint
- Add fsync_thread_running flag to guard persist_stop when thread not started
- Add zlib_dep to controller and unit test meson targets
- 3 new TDD tests: init_creates_directories, wal_async_writes_entry,
  checkpoint_type_rejected — all GREEN

src/controller/meson.build

@@ -16,6 +16,7 @@ controller_src = [
   'worker.c',
   'apiserver.c',
   'myconf.c',
+  'persist.c',
 ]
 
 incdir = include_directories('.', '../include', '../lib',
@@ -24,13 +25,16 @@ incdir = include_directories('.', '../include', '../lib',
 # cc = meson.get_compiler('c')
 # math_dep = cc.find_library('m', required : false)
 
+zlib_dep = dependency('zlib')
+
 controller_deps = [
   libxce_dep,
   librtti_dep,
   c_capnproto_dep,
   math_dep,
   tinylog_dep,
-  thread_dep
+  thread_dep,
+  zlib_dep,
 ]
 
 controller = executable('xce_controller',
@@ -65,12 +69,16 @@ xce_test = executable('xce_test',
 controller_unit_test_src = [
   'testsuite/controller_test.c',
   'testsuite/test_persist_types.c',
+  'testsuite/test_persist_wal.c',
+  'persist.c',
 ]
 
 controller_unit_test_deps = [
   libxce_dep,
   unity_dep,
   tinylog_dep,
+  thread_dep,
+  zlib_dep,
 ]
 
 controller_unit_test = executable('controller_unit_test',