docs(codec-eval): make the API quick-start compilable (callback signature + report shape)

[lilith]

Why

An "insulated external developer" usability test (given only the README) found the methodology/fair-comparison content excellent, but the API quick-start was uncompilable. Concretely:

• The encode-callback was written against fictional types: Box::new(|image, request| my_codec::encode(image, request.quality)) with no real image/request/return types shown.
• The example called session.evaluate_corpus("./test_images") — evaluate_corpus does not exist in the crate. The real method is evaluate_image(name, ImageData).
• The returned report was never read — no field of ImageReport/CodecResult was shown, so a reader couldn't tell how to get a metric or a byte count out.
• quality_levels semantics were ambiguous: encoder quality (0-100) vs. target SSIMULACRA2 scores. The README's own "don't compare by quality" methodology made this genuinely confusing.
• The install lines claimed git-only, but the library is published on crates.io as codec-eval = "0.3.2".

What this changes (README only, + a CHANGELOG note)

Rewrites the quick-start so every codec-eval symbol resolves, verified by compiling a faithful skeleton (cargo check, clean):

• Real callback signature — Box::new(|image: &ImageData, request: &EncodeRequest| -> Result<Vec<u8>> { ... }), matching EncodeFn in src/eval/session.rs:181. Shows image.to_rgb8_vec() / image.width() / image.height() and returns encoded bytes.
• Correct entry point — session.evaluate_image("test.png", reference)? returning an ImageReport (src/eval/session.rs:368), with a note that it takes an already-decoded ImageData (the harness doesn't read files) and a pointer to Corpus::get_dataset(...) for the built-in corpora.
• Report-read snippet — iterates report.results and prints codec_id, quality, file_size, bits_per_pixel, metrics.ssimulacra2, metrics.dssim (fields from src/eval/report.rs), plus write_image_report.
• quality_levels clarified — documented inline as encoder quality settings (0-100) drawn into request.quality (a codec-specific dial), not target metric scores (EncodeRequest::quality is f64, "0-100, codec-specific interpretation", src/eval/session.rs:153).
• Accurate install — cargo add codec-eval (crates.io) as the primary line; git for unreleased; the companion codec-eval-cli flagged git-only.

Note: EncodeRequest is not re-exported at the crate root, so the example imports it via its real path codec_eval::eval::session::EncodeRequest (flagged in a comment). The same src/lib.rs doc-comment quick-start has the analogous Ok(encoded_bytes)-with-undefined-vars shape; left out of scope to keep this diff README-only, but worth a follow-up.

Methodology / fair-comparison / R-D / viewing-conditions content is unchanged.

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️