Finalize v1 high-level image API and release

[justin13888]

• Semver review of the final surface per docs/V1_DESIGN.md: format-keyed DecodeOptions/EncodeOptions, encode_image{,_to_vec,_to_writer}, RgbImage wrapper, RawFile (+format()), registry, prelude, error model (Gamut{context,source}, HwDecoderUnavailable{codec,reason}; revisit context when gamut-core: structured diagnostic context in Error gamut#254 lands).
• Rustdoc pass; CHANGELOG; cut 1.0.0-rc.
• Explicit release decision: git-pinned gamut cannot be published to crates.io — v1 is a git-consumable release until gamut publishes (crates.io release deferred by owner decision).
• Depends on: everything else in the tracking issue.