gamut: forward gamut-{jpeg,jxl,heic}'s `metadata` feature from the umbrella's own `metadata`

[justin13888]

#420 gives gamut-jpeg, gamut-jxl and gamut-heic an optional metadata Cargo feature that turns on the typed blocks() / metadata() accessors and the encoders' with_metadata. The umbrella crate's own metadata feature currently enables only the metadata crates themselves:

metadata = [
    "dep:gamut-metadata",
    "dep:gamut-exif",
    "dep:gamut-xmp",
    "dep:gamut-icc",
    "dep:gamut-iptc",
    "dep:gamut-ifd",
]

so a consumer who depends on gamut with features = ["jpeg", "metadata"] gets the facade types and the JPEG codec, but not the wiring between them: JpegMetadata::metadata() is compiled out. Reaching it means depending on gamut-jpeg directly with its metadata feature, which defeats the umbrella.

Ask: add the weak, per-format forwards so the umbrella's metadata feature implies the format crates' when — and only when — that format is enabled:

metadata = [
    ...,
    "gamut-jpeg?/metadata",
    "gamut-jxl?/metadata",
    "gamut-heic?/metadata",
]

and extend it to png / webp / avif / tiff as #510 wires them.

Checks that must stay green: mise run check-ffi-features (no new feature name is introduced, so gamut-ffi's table should already mirror it — confirm), mise run check-release-deps, and a cargo test -p gamut --all-features that actually exercises one forwarded accessor so the edge cannot rot silently.

Split out of #420: that branch's manifest is the format crates, and this is a change to crates/gamut/Cargo.toml. Related: #543 (the same shape for gamut-exif's describe).

Refs #420, #416.