feat(webp)!: migrate WebP to gamut-webp

[justin13888]

Summary

• replace the in-repo libwebp-sys FFI codec/mux wrapper with crates.io gamut-webp 0.3.1 for pure-Rust VP8/VP8L decode and encode
• route WebP EXIF/XMP/ICC chunks through gamut-webp and gamut-metadata::MetadataBlock, removing the manual RIFF EXIF scanner
• remove the C dependency, libwebp feature aliases, backend-specific error, and legal notice; publish webp/gamut at registry version 0.3
• preserve interoperability with a tracked WebP fixture produced by the previous libwebp backend and add WebP encode/decode + metadata round-trip coverage

Breaking API changes

• LibwebpDecodeConfig → WebpDecodeConfig
• LibwebpEncodeConfig → WebpEncodeConfig
• WebP quality changes from f32 to u8; the libwebp-only method and near_lossless fields are removed (future tuning remains upstream in gamut-webp: encoder knobs — effort/method and near-lossless gamut#261)
• EncodeError::WebP, webp-decode-libwebp, and webp-encode-libwebp are removed

Performance

Full Criterion runs on the same Ryzen 7 7800X3D host, using the 512×512 synthetic lossy round trip at quality 75:

Benchmark	libwebp 0.14.2	gamut-webp 0.3.1
encode	7.40 ms	9.97 ms
decode	766 µs	4.88 ms

The decode comparison uses each backend's own encoded output. These numbers are recorded in docs/BENCHMARKS.md as the new upstream-first baseline.

Validation

• cargo test --workspace --features rawshift-image/full
• cargo clippy --workspace --all-targets --features rawshift-image/full -- -D warnings
• cargo doc --workspace --no-deps --features rawshift-image/full
• cargo publish --dry-run -p rawshift-image
• direction-only checks for webp-decode, webp-encode, and webp
• full Criterion run for decode, demosaic, and pipeline benches (HEIC/AVIF hardware benches skipped cleanly because their external encoder tools are unavailable)

Closes #24