fix: teardown UAF in Track block cache and headless test/render environment

- Track: new InputDisconnectedEvent trims blocks_/block_array_indexes_
  when a block edge is removed outside the Track's own operations
  (block deletion, undo commands detaching a whole track). Previously
  blocks_ kept dangling pointers and Project teardown crashed in
  track_length() (ASan heap-use-after-free in oakengine_timeline_edit /
  oakengine_sync). The persistent array map is intentionally not
  rewritten so undo of remove_track can re-attach the clips; replace_block
  guards the handler with ignore_block_disconnect_
- filefunctions: honor OAK_CONFIG_DIR to redirect the configuration
  root; QStandardPaths ignores XDG_* on macOS, so the engine tests read
  the real user config (a stale 0.1.x file) and failed frame-rate
  assertions. All engine tests now set OAK_CONFIG_DIR to their tmpdir
- init test: chdir to the fixture directory before loading
  project_with_footage.ove so the engine's moved-project footage
  relocation does not rewrite the stored relative filename
- renderworkerpool: do not let the render worker inherit
  QT_QPA_PLATFORM=offscreen from a headless host process; the worker
  needs a real platform GL context and exited immediately otherwise,
  failing oak_cli_transcode

engine/tests/oakengine_events_test.cpp

@@ -922,6 +922,7 @@ TEST(OakEngineEvents, Main)
 	EXPECT_TRUE(setenv("XDG_CONFIG_HOME", g_tmpdir, 1) == 0);
 	EXPECT_TRUE(setenv("XDG_CACHE_HOME", g_tmpdir, 1) == 0);
 	EXPECT_TRUE(setenv("XDG_DATA_HOME", g_tmpdir, 1) == 0);
+	EXPECT_TRUE(setenv("OAK_CONFIG_DIR", g_tmpdir, 1) == 0);
 #endif
 
 	EXPECT_TRUE(oakengine_init(OAKENGINE_INIT_HEADLESS) == OAKENGINE_OK);