Implement functional VP8 encoder with spec-compliant bitstream output

[Copilot]

Implements a working VP8 encoder capable of producing valid bitstreams for keyframes using intra-prediction. The encoder was initially producing frames that failed decode with VPX_CODEC_MEM_ERROR due to bitstream format violations.

Implementation

Core Encoder (vp8_encoder.cs)

• Frame header encoding (uncompressed + compressed per VP8 spec)
• Keyframe encoding with DC prediction mode
• Forward DCT transform and quantization
• Y2 block encoding with Walsh-Hadamard transform for 16x16 prediction modes
• Coefficient tokenization with boolean entropy encoding
• Two-pass macroblock encoding: modes first, then token data

Bitstream Format Fixes

• Dimensions encoded as (width-1, height-1) per spec
• Y mode uses 3-bit DC_PRED encoding (tree: 1,0,0)
• UV mode uses 1-bit DC_PRED encoding (tree: 0)
• Proper plane types: Y2=1, Y_AC=0, UV=3
• Added mb_no_coeff_skip flag before macroblock data
• Correct token/EOB encoding for coefficient blocks

Integration (VP8Codec.cs)

• Wired encoder into EncodeVideo() method
• Configurable quantizer (0-63) for quality control

Usage

var encoder = new VP8Encoder(640, 480);
encoder.SetQuantizer(10);  // Lower = better quality
byte[] encoded = encoder.EncodeFrame(i420Data);

// Or via VP8Codec
var codec = new VP8Codec();
byte[] encoded = codec.EncodeVideo(width, height, pixels, pixelFormat, codec);

Limitations

• Keyframes only (no inter-frame prediction)
• DC prediction mode (simplest intra mode)
• Single partition

Files Changed

• src/vp8_encoder.cs - Complete encoder implementation
• src/VP8Codec.cs - Integration
• src/onyxe_int.cs - Encoder structures
• src/fdctllm.cs - Forward DCT
• src/quantize.cs - Quantization
• test/VP8.Net.UnitTest/VP8EncoderWorkingTest.cs - Tests
• Supporting documentation files

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