feat: add WebP (RIFF/WEBP) image EXIF support

[mindeng]

Summary

Adds WebP image support to nom-exif: detects RIFF/WEBP files and extracts EXIF/GPS from the EXIF chunk via the unified parse_exif / read_exif (sync + async) APIs. Scope is EXIF-only (no XMP).

• MIME detection (src/file.rs): new MediaMimeImage::Webp + check_webp ("RIFF"…"WEBP"), wired into the detection chain.
• Pure RIFF chunk walker (src/webp.rs): webp::extract_exif(state, buf) walks the chunk stream, skipping large image-bitstream chunks via ClearAndSkip, returning the EXIF payload (raw TIFF). Bounds the walk by the RIFF size field (WebP has no terminator chunk) and threads the remaining-bytes bound across skips via ParsingState::WebpPastHeader(usize). Strips a stray Exif\0\0 marker if an encoder wrote one.
• Generic dispatch integration (src/exif.rs): rides the existing extract_exif_with_mime path — reuses the sync and async drivers with zero async-specific code. New MalformedKind::WebpChunk.
• Streaming EOF tolerance (src/parser.rs): small-file tolerance widened from Png to Png | Webp.

Design notes

WebP EXIF sits after the image bitstream, so the walker must skip potentially-large VP8 /VP8L/ANMF chunks to reach it — handled by ClearAndSkip with the RIFF-size bound carried across the skip. A no-EXIF WebP terminates cleanly with Error::ExifNotFound rather than reading to EOF.

Full design + implementation plan under docs/superpowers/.

Test Plan

• cargo test — 427 passed, 0 failed (default features)
• cargo test --features tokio — 432 passed, 0 failed
• cargo fmt --check clean; cargo clippy --all-targets --all-features no issues
• 18 WebP-specific tests: 12 walker unit tests (Need/ClearAndSkip resume, prefix strip, odd-size padding, RIFF-bound, overflow, bad/truncated header), 2 MIME-detection tests (incl. RIFF/WAVE negative), 4 end-to-end (memory sync, streaming sub-buffer, no-EXIF → ExifNotFound, async)

[codecov]

Codecov Report

❌ Patch coverage is 95.15152% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.69%. Comparing base (d0b1412) to head (4012cf9).

Files with missing lines	Patch %	Lines
src/webp.rs	94.28%	12 Missing ⚠️
src/parser.rs	96.66%	3 Missing ⚠️
src/error.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
+ Coverage   90.52%   90.69%   +0.16%     
==========================================
  Files          38       39       +1     
  Lines        8593     8921     +328     
==========================================
+ Hits         7779     8091     +312     
- Misses        814      830      +16     

Flag	Coverage Δ
nom-exif	90.69% <95.15%> (+0.16%)	⬆️

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