v0.0.4

Other

• drop stale REGISTRARS / with_all_features intra-doc links
• drop dead linkme dep
• wire Encoder trait + registry, spiral+diamond ME
• registry calls: rename make_decoder/make_encoder → first_decoder/first_encoder
• auto-register via oxideav_core::register! macro (linkme distributed slice)
• unify entry point on register(&mut RuntimeContext) (#502)

Added

• Encoder Encoder trait implementation and registry wiring.
  H261RegistryEncoder wraps H261Encoder and implements the
  oxideav_core::Encoder trait (send_frame / receive_packet / flush).
  make_encoder(params) is now registered in register_codecs via
  .encoder(encoder::make_encoder) so callers can use
  reg.first_encoder(&params) with H.261.
  The factory accepts params.width / params.height (QCIF or CIF) and
  derives an initial GQUANT from params.bit_rate via an empirical
  bits-per-frame-at-Q1 model. The per-GOB MQUANT rate controller then
  nudges ±1 step dynamically. At 64 kbit/s QCIF the factory selects
  QUANT ≈ 9, delivering ≥ 45 dB PSNR_Y on smooth gradient content.

• Spiral + diamond motion estimation replaces the prior flat full-window
  scan. The new search evaluates concentric ring boundaries innermost-first
  with early termination when two consecutive rings show no improvement,
  then refines with an 8-connected neighbourhood around the winner. On
  static/smooth content this saves evaluating the outer rings (~80 % of the
  961-point full scan) while maintaining quality; on complex motion it falls
  back to the full ±15 range. A compact 3-tap diamond fallback catches any
  one-pel miss at ring boundaries.

Changed

• Updated lib.rs module docstring — the crate is no longer decode-only.
• README feature matrix updated: MC encode, loop-filter-with-RDO encode,
  per-GOB rate control, and registry encoder rows added.

Tests added

• register_tests::register_via_runtime_context_installs_encoder_factory
• register_tests::encoder_factory_qcif_defaults
• register_tests::encoder_factory_cif
• register_tests::encoder_factory_rejects_bad_dimensions
• encoder::tests::make_encoder_derives_quant_from_bit_rate — encodes 8 QCIF
  frames via the Encoder trait at 64 kbit/s target; asserts avg PSNR_Y ≥ 35 dB.
• ffmpeg_roundtrip::registry_encoder_qcif_roundtrip — encodes 4 QCIF frames
  via the Encoder trait, feeds the stream to ffmpeg, asserts clean decode.
• ffmpeg_roundtrip::encoder_psnr_vs_source_at_default_quant — encodes 8
  QCIF frames of a moving gradient and verifies avg PSNR_Y ≥ 32 dB after
  ffmpeg cross-decode.