feat(core): add structured error diagnostics

[justin13888]

Summary

• add stable ErrorKind classification and an owned Diagnostic context wrapper while preserving the existing allocation-free Error variants
• propagate crate origin, parser-relative byte offsets, and dynamic details through the active producer chain; classify fallback behavior by kind and preserve accepted backend errors unchanged
• document the Rust and planned FFI diagnostic contract, including compatibility and traversal semantics

Top-down scope

• Public foundation: gamut-core::Error, ErrorKind, and Diagnostic, re-exported through gamut::core
• Shared producers: core, color, tonemap, IFD, ISOBMFF, RIFF, and XMP
• Format producers: AV1, AVIF, DNG, HEIC, JPEG, JXL, PNG, TIFF, and WebP
• Consumers: semantic fallback checks and codec-ABI adapters now use stable classification while retaining exact external backend failures
• Boundary documentation: the FFI design/header/status mapping describes kind plus contextual text without expanding the current FFI implementation

AV2 and VVC remain declaration-only stubs. The dependency-free codec ABI and separate metadata error domains stay unchanged except where XMP converts into the core error type. CLI and WASM behavior are outside this change.

Compatibility

The three legacy variants remain source-compatible and allocation-free. Structured context is additive through the non-exhaustive Context variant; decorators are first-write-wins and preserve the source chain.

Validation

• mise run fmt-check
• mise run lint
• mise run test
• mise run coverage — 97.22% lines
• mise run check-msrv
• mise run check-ffi
• mise run check-ffi-features
• mise run check-ffi-header
• mise run check-ffi-seams
• mise run check-release-deps
• mise run check-commits
• targeted mutation rechecks caught every previously missed or timed-out coordinate, including the hardened CCITT non-progress guard
• GitHub incremental mutation gate passed the complete PR diff

Closes #254