feat: transcode WXGF image attachments

[richardzone]

What changed

• Adds a WXGF/WXAM helper that finds the largest HEVC partition in WeChat 4 image containers.
• Makes wx extract automatically transcode decoded WXGF/HEVC images to JPG through ffmpeg.
• Adds wx extract --raw for callers that need to keep the decoded WXGF/HEVC container.
• Documents the WeChat cache behavior where unopened images may only have _t.dat thumbnails locally.

Why

WeChat 4 can store image attachments as private wxgf containers. wx extract could decrypt the .dat file but still write a container that normal image viewers and OCR tools could not read. This keeps normal image outputs usable by default while preserving a raw escape hatch.

Implementation notes

• No Python or native image decoder dependency is added.
• WXGF parsing is done in Rust.
• ffmpeg is invoked through temp input/output files instead of stdin/stdout pipes to avoid pipe backpressure or orphaned child process issues.
• WX_FFMPEG can override the ffmpeg path.

Validation

• Subagent code review performed; pipe/backpressure and partition length concerns were addressed.
• cargo check --locked
• cargo test --locked