gamut-av1 and gamut-avif publish crate descriptions that their own source contradicts

[justin13888]

Found while correcting the README crates table for #425 (PR #526). Both are outside that PR
boundary, which is frozen to README.md, AGENTS.md one wording, the guard and its wiring — and
both are published text: a Cargo.toml description is what crates.io and cargo search show,
and a crate-level module doc is the first screen of docs.rs.

1. gamut-av1 describes itself as encoder-only, over a decode-by-default manifest

crates/gamut-av1/Cargo.toml:3:

description = "AV1 still-image (intra-frame) encoder — the standalone codec layer beneath AVIF."

crates/gamut-av1/Cargo.toml:19:

default = ["decode"]

The decoder is not an opt-in extra a description may reasonably omit — it is on by default, it is
crates/gamut-av1/src/decode/ (4223 lines), and src/lib.rs re-exports Av1Decoder,
DecodeLimits, FrameHeader and friends behind that feature. src/lib.rs:1 opens with
"AV1 image encoder." for the same reason.

Anyone reading crates.io concludes gamut-av1 cannot decode. It can, unless they turn the feature
off.

2. gamut-avif lists as deferred what its own source already ships

crates/gamut-avif/src/lib.rs, under Deferred, planned:

alpha / RGBA encoding, 10/12-bit and 4:2:0/4:2:2 chroma subsampling, … the pure-Rust AV1
codestream decoder (which will make Av1StillDecoder optional) …

Contradicted twice by the crate itself:

• src/lib.rs:4-7, forty lines above: AvifEncoder implements gamut_core::EncodeImage for
  Rgba8, Rgb16 and Rgba16. Alpha and high bit depth are the encode surface, not a plan.
• The pure-Rust AV1 codestream decoder exists: gamut-av1 default = ["decode"], and
  crates/gamut-av1/src/decode/mod.rs:3-4 describes itself as "the pure-Rust software tail behind
  gamut_avif::Av1StillDecoder". gamut-avif already has a normal dependency on gamut-av1 with
  that feature on; what is missing is the wiring, not the decoder.

So the module doc tells a reader that AVIF decode requires them to bring a third-party AV1
decoder (dav1d, a platform decoder), while the workspace ships one.

Scope

• Rewrite gamut-av1 Cargo.toml description and src/lib.rs:1 to state encoder and
  decoder, with the decoder scope bounded as src/decode/mod.rs:20-22 bounds it.
• Reconcile gamut-avif "Deferred, planned" list with its own encode surface, and restate the AV1
  decode position as "gamut-av1 ships one; gamut-avif does not yet register it as the implicit
  tail behind Av1StillDecoder" if that is the intent.
• Related but distinct: STATUS.md drift: gamut-dsp lists a shipped module as deferred, gamut-avif contradicts itself #545 covers the same class in two STATUS.md files.

Acceptance

No crate Cargo.toml description or crate-level module doc names a capability direction the same
crate manifest or source contradicts.

Refs #425, #526, #545.