Feature tree + compile-boundary CI rework post-migration

[justin13888]

• Collapse the 5-tier feature hierarchy: tier-4 implementation features disappear (gamut is the backend); final tree = bundles → formats → directions (mapping to gamut crate features) + serde + hw/hw-* + full (all formats + serde + experimental + hw) — see docs/V1_DESIGN.md tables.
• Compile-boundary CI: build the invalid feature×target combos (e.g. hw-vaapi on macOS) expecting compile_error!; build full on every tier-1 target (docs/SUPPORT.md) expecting success.
• Delete build.rs bindgen/cmake/cc machinery as each vendored C path goes (jpegli, libaom, libjxl); keep cfg aliases + boundary verification; update [lints.rust] check-cfg.
• Update docs.rs feature list, README feature-flag section, justfile, lefthook, license inventory (libheif/aom/jpegli notices removed).
• CI matrix = docs/SUPPORT.md targets incl. macOS runner; MSRV in lockstep with gamut.
• Depends on: rolling with Migrate DNG decode/encode to gamut-dng #21–Create rawshift-hwdec (HEVC+AV1); HEIC on gamut-heic; drop libheif-rs #27; final pass after Migrate JPEG to gamut #31/Migrate PNG decode to gamut-png #32/AVIF decode via gamut-avif container + hardware AV1 #33.