build/ci: green builds and tests on all three platforms

Compiler hygiene (all platforms):
- Silence warnings across the tree: missing override, -Wreorder ctor
  init, -Wshadow, -Wsign-compare, missing switch cases, unused
  functions/captures, Qt 6.11 deprecations (QMouseEvent/QDropEvent
  accessors, qAsConst, Q_FOREACH over non-shared containers,
  AA_UseHighDpiPixmaps) and the .bak/ backup tree removal.
- Fix regressions from the cleanup: missing clip decls in
  capi/timeline.cpp, plugin.cpp rename fallout, panel setFocus
  ambiguity, QGraphicsItem::pos vs event->position(), duplicate
  k_push_button case, boolean test variable.

Windows:
- Qt portability: CommandLineParser is_set/add_option, QTimeZone
  systemTimeZone (QTimeZone::LocalTime is 6.11-only), k_progress_* enum.
- Linking: stop adding oakengine to OLIVE_LIBRARIES (import lib plus
  oakengine-obj caused multiple definitions); add OAKENGINE_STATIC so
  internal consumers no longer reference __imp_* stubs.
- oakengine.ver: export olive::Renderer typeinfo so liboakgl.so can be
  dlopened (Linux), DynamicRenderer no longer dlcloses backend libraries
  (crash in RenderManager's dtor calling into unmapped memory).
- OTIO runtime: copy DLLs next to every binary on Windows instead of
  relying on PATH (0xc0000135 in gtest discovery).
- Headless GL: the runner only has GDI OpenGL 1.1, killing every render
  worker. Deploy Mesa llvmpipe as opengl32sw.dll (Qt's software-GL
  channel) with QT_OPENGL=software, and let QT_OPENGL override the
  AA_UseDesktopOpenGL default. ExportTask fails fast after 8 consecutive
  undelivered frames instead of segfaulting or grinding forever;
  FFmpegEncoder::write_frame tolerates null frames.
- Tests: GetTempPathA+PID temp dirs, GetLongPathNameA for 8.3 names,
  forward-slash normalization when comparing project filenames.

Linux:
- Install libshaderc-dev so oakvulkan compiles GLSL (Vulkan tests).
- Accept UNORM floor-or-round (63/64) in the blit ping-pong test.
- Skip MainWindow construction test on the offscreen QPA (cannot paint
  QOpenGLWidget).

Also: oak_cli_transcode gets a 300s ctest timeout, worker logs GL
context version and LoadGraph/render_frame stages, and
docs/plans/eliminate-event-bridge-issues.md (English translation).

app/dialog/export/export.cpp

@@ -61,7 +61,7 @@ namespace
 
 // pix_fmt string (e.g. "yuv420p") to its index in the codec's supported
 // list; 0 (the codec's preferred format) when absent.
-int pix_fmt_index(int codec, const QString &pix_fmt)
+[[maybe_unused]] [[maybe_unused]] int pix_fmt_index(int codec, const QString &pix_fmt)
 {
 	if (pix_fmt.isEmpty()) {
 		return 0;
@@ -87,7 +87,7 @@ Rational export_viewer_length(const OakEngineNode *viewer)
 
 // OakEngineEncodingParams (assembled by the dialog) -> facade POD. One-to-one with
 // oak_export_options_ex; see oakengine/exporter.h for the field docs.
-oak_export_options_ex params_to_ex(const OakEngineEncodingParams *p)
+[[maybe_unused]] [[maybe_unused]] oak_export_options_ex params_to_ex(const OakEngineEncodingParams *p)
 {
 	oak_export_options_ex o = {};
 
@@ -939,7 +939,6 @@ OakEngineEncodingParams *ExportDialog::generate_params() const
 		const int vh = static_cast<int>(video_tab_->height_slider()->get_value());
 		const Rational tb = get_selected_timebase();
 		const int pix_fmt = video_tab_->pixel_format_field()->get_pixel_format();
-		const int ch_count = oakengine_video_params_internal_channel_count();
 		const Rational par = video_tab_->pixel_aspect_combobox()->get_pixel_aspect_ratio();
 		const int interlace = video_tab_->interlaced_combobox()->get_interlace_mode();