feat(oakcodec): hardware video decoding by default on all platforms

FFmpeg 8 removed the standalone hardware decoders (h264_videotoolbox/
vaapi/nvdec/d3d11va no longer exist in its configure) — hardware decode
now only exists as a hwaccel attached to the software decoder. The new
oakcodec::hwdecode module therefore opens the regular decoder with the
platform's hardware device context attached (VideoToolbox on macOS,
VA-API then NVDEC on Linux, D3D11VA then NVDEC on Windows): FFmpeg
engages the matching hwaccel, decodes into hardware surfaces, and we
transfer them to system memory (NV12/P010) ahead of swscale.

- HardwareDecoding config switch, default ON by mandate; a checkbox in
  Preferences > Rendering (EN/ZH); device creation failure skips to the
  next candidate and finally to software; a decode-time failure on a
  hardware session reopens it as software and retries once.
- hw_decoder_name() observability hook plus a HW_TRANSFERS counter so
  tests can prove the hwaccel really engaged (not silently software).
- Verification: demo.mp4 H.264 decodes through VideoToolbox with a
  transferred hardware surface, and the pixels match the software
  decode within 0.05; switch off forces software.
- build-ffmpeg.sh also enables nvdec when ffnvcodec headers exist.

src/dialogs.rs

@@ -95,6 +95,7 @@ pub struct PreferencesContent {
 	theme: Entity<ComboBox>,
 	cache_dir: Entity<PathField>,
 	use_proxy: Entity<CheckBox>,
+	hw_decode: Entity<CheckBox>,
 	proxy_divider: Entity<ComboBox>,
 	snapshot_interval: Entity<SpinBox>,
 	transition_length: Entity<SpinBox>,
@@ -251,6 +252,30 @@ impl PreferencesContent {
 			combo.set_selected(Some(divider_selected), cx)
 		});
 
+		// --- 渲染 Rendering: hardware decoding switch -------------------
+		// Default ON (user mandate); off forces software decoding.
+		let hw_decode = cx.new(|cx| {
+			CheckBox::new(
+				9,
+				if config_get_bool("HardwareDecoding", true) {
+					CheckState::Checked
+				} else {
+					CheckState::Unchecked
+				},
+				window,
+				cx,
+			)
+			.with_label(i18n::tr("preferences.hwdecode.enable"))
+		});
+		cx.subscribe(&hw_decode, |_this, check, event: &CheckBoxEvent, cx| {
+			if let CheckBoxEvent::Toggled { state, .. } = event {
+				let enabled = *state == CheckState::Checked;
+				config_set_bool("HardwareDecoding", enabled);
+				check.update(cx, |check, cx| check.set_state(*state, cx));
+			}
+		})
+		.detach();
+
 		// --- 项目 Project: snapshot interval + default transition ----------
 		let snapshot_interval = cx.new(|cx| {
 			let current =
@@ -342,6 +367,7 @@ impl PreferencesContent {
 			theme,
 			cache_dir,
 			use_proxy,
+			hw_decode,
 			proxy_divider,
 			snapshot_interval,
 			transition_length,
@@ -499,6 +525,7 @@ impl Render for PreferencesContent {
 				i18n::tr("preferences.backend").into(),
 				self.backend.clone(),
 			))
+			.child(self.hw_decode.clone())
 			// 缓存 Cache
 			.child(section_header(&colors, i18n::tr("preferences.section.cache").into()))
 			.child(form_row(

src/i18n.rs

@@ -434,6 +434,7 @@ const EN: &[(&str, &str)] = &[
 	("preferences.section.project", "Project"),
 	("preferences.section.audio", "Audio"),
 	("preferences.backend", "Renderer backend"),
+	("preferences.hwdecode.enable", "Hardware-accelerated video decoding (VideoToolbox / VA-API / NVDEC / D3D11VA)"),
 	("preferences.backend.placeholder", "Select a backend…"),
 	("preferences.language", "Language"),
 	("preferences.language.placeholder", "Select a language…"),
@@ -899,6 +900,7 @@ const ZH: &[(&str, &str)] = &[
 	("preferences.section.project", "项目"),
 	("preferences.section.audio", "音频"),
 	("preferences.backend", "渲染后端"),
+	("preferences.hwdecode.enable", "硬件加速视频解码（VideoToolbox / VA-API / NVDEC / D3D11VA）"),
 	("preferences.backend.placeholder", "选择一个后端…"),
 	("preferences.language", "语言"),
 	("preferences.language.placeholder", "选择语言…"),