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).

tests/gtest/mainwindow_test.cpp

@@ -3,6 +3,7 @@
 #include <QProgressBar>
 #include <QOffscreenSurface>
 #include <QOpenGLContext>
+#include <QGuiApplication>
 #include <QSignalSpy>
 #include <QTest>
 #include <QXmlStreamReader>
@@ -174,6 +175,14 @@ TEST(MainWindow, ConstructsOffscreenWithPanelsAndMenus)
 	// runners on the offscreen QPA), constructing it crashes in GL code
 	// ("QOpenGLFunctions created with non-current context"). Probe first and
 	// skip where GL is unavailable.
+	// MainWindow instantiates viewer panels containing QOpenGLWidget. The
+	// offscreen QPA cannot paint QOpenGLWidget at all ("QOpenGLWidget is not
+	// supported on this platform") — pumping events then crashes inside Qt's
+	// backing store flush. Skip there regardless of whether a bare
+	// QOpenGLContext can be created.
+	if (QGuiApplication::platformName() == QStringLiteral("offscreen")) {
+		GTEST_SKIP() << "offscreen QPA cannot paint QOpenGLWidget";
+	}
 	QOffscreenSurface probe_surface;
 	probe_surface.create();
 	QOpenGLContext probe_context;