v0.9.0: Phase 7 Wave 5 -- Multimedia Framework

v0.9.0: Phase 7 Wave 5 -- Multimedia Framework

Phase 7 Wave 5 adds a complete audio subsystem and video framework to VeridianOS. The audio stack uses fixed-point 16.16 integer math for mixing (no floating-point in kernel), lock-free SPSC ring buffers for transport, and targets VirtIO-Sound hardware. The video framework provides image decoding (TGA, QOI), frame scaling, color space conversion, and a media player with tick-based timing. 18 files changed, ~3,900 new lines (13 new, 5 modified).

Audio Subsystem (Sprints A-1 through A-5)

• Audio mixer (audio/mixer.rs): Fixed-point 16.16 arithmetic with saturation, multi-channel mixing with per-channel volume (AtomicU32) and mute (AtomicBool), master volume control
• Ring buffer transport (audio/buffer.rs): Lock-free SPSC ring buffer with AtomicU32 read/write positions, typed write_samples(&[i16])/read_samples(&mut [i16]) interface
• WAV parser (audio/wav.rs): RIFF/WAVE chunk iteration, 8/16/24/32-bit PCM with conversion utilities
• Output pipeline (audio/pipeline.rs): Mixer-to-output buffering with underrun tracking and drain support
• VirtIO-Sound driver (audio/virtio_sound.rs): PCI scan for 0x1AF4:0x1059, MMIO register access, PCM stream configuration (set_params/prepare/start/stop)
• Audio client API (audio/client.rs): Stream lifecycle (create/play/pause/stop/set_volume), automatic mixer channel registration
• Audio syscall stubs: 8 syscalls (AudioOpen=320 through AudioPause=327)
• Shell commands: play <file.wav> (WAV parse + PCM queue), volume [0-100] [stream_id]

Video Framework (Sprints V-1 through V-3)

• Pixel formats (video/mod.rs): XRGB8888, ARGB8888, RGB888, RGB565, BGR888, BGRX8888, Gray8 with full conversion
• Scaling and color (video/framebuffer.rs): Nearest-neighbor + bilinear (fixed-point 8.8 interpolation), BT.601 YUV/RGB integer conversion, alpha blending
• Image decoders (video/decode.rs): TGA (uncompressed + RLE, 24/32-bit BGR/BGRA), QOI (full spec: INDEX/DIFF/LUMA/RUN/RGB/RGBA)
• Media player (video/player.rs): RawVideoStream with tick-based frame timing, playback state machine (Stopped/Playing/Paused/Finished)
• Image viewer (desktop/image_viewer.rs): TGA and QOI support alongside existing PPM/BMP

Documentation

• New: docs/AUDIO-SUBSYSTEM.md -- audio architecture, mixer, ring buffer, VirtIO-Sound, pipeline
• New: docs/VIDEO-FRAMEWORK.md -- pixel formats, scaling, decoders, media player

Build & Testing

• All 3 architectures: zero warnings, zero errors
• 29/29 boot tests passing on x86_64, AArch64, RISC-V
• 33+ new unit tests for mixer, ring buffer, WAV parser, pipeline, pixel formats, scaling, decoders, player