refactor(image): isolate format package boundaries

[justin13888]

Summary

• split all 17 image formats into independently publishable rawshift-image-* leaf crates
• add rawshift-image-core capability/error/image contracts plus focused metadata, IFD, and lossless-JPEG support crates
• preserve the existing rawshift-image and rawshift APIs and feature facade while keeping processing, transforms, camera data, and APV central
• expose direct default decoders for every implemented leaf and default encoders for JPEG, PNG, WebP, AVIF, JXL, and DNG
• integrate the gamut-webp migration from master into the extracted, safe-Rust rawshift-image-webp leaf

Design

Format leaves depend on shared contracts instead of the aggregate crate, so the dependency graph remains acyclic. The aggregate crate uses default-features = false leaf dependencies and forwards its existing direction features. Direct leaf dependencies default to decode support. Gamut remains the upstream implementation wherever available; no gamut workaround was added.

Validation

• cargo fmt --all -- --check
• just clippy
• just test
• just test-all
• cargo test -p rawshift-image-webp --no-default-features --features decode,encode,serde
• WebP decode-only, encode-only, combined, and serde feature checks
• focused facade WebP export and libwebp-fixture interoperability tests
• image-only and video-only builds plus dependency-isolation assertions
• full-feature rawshift-image and rawshift checks
• strict workspace rustdoc build with warnings denied
• CI compile-boundary matrix for valid and invalid feature/target combinations

Risks and rollout

This is a breaking v1 package/API refactor; 0.x source compatibility is explicitly a non-goal for the release. The merge preserves the WebP API and behavior now present on master while relocating its implementation behind the new leaf boundary.

Issue

Closes #14