feat(corruption-corpus): structural-corruption generator, decoupled from the codec-corpus fetcher

[lilith]

What

Introduces corruption-corpus — a standalone workspace-member crate that
generates a held-out falsification set for a perceptual metric's negative
tail: localized structural defects no honest encoder would produce (a channel
swap, a dropped block, an off-by-one edge) that a faithful metric must rank
below an honestly-lossy encode of the same reference.

The gate every corpus entry asserts:

score(reference, corruption) < score(reference, honest_lq_anchor)   // JPEG q20 / q10

Built per the spec at zensim/docs/structural_corruption_corpus_spec_2026-05-27.md.

Structure — generator decoupled from fetcher

The structural-corruption generator now lives in its own crate
(crate/corruption-corpus/, package corruption-corpus, lib
corruption_corpus), decoupled from the codec-corpus fetcher (which
downloads / caches / serves test-image datasets). The module was self-contained
— it imported only image, serde, and its own prng (SplitMix64), used none
of codec-corpus's download/registry code, and the fetcher used none of it — so
the split is a clean move; no generator logic changed. Consequences:

• codec-corpus no longer depends on the image crate in its default build;
  the driver feature + the corruption_corpus example moved to the new crate.
• The example binary still lands in the shared workspace target/
  (crate/target/release/examples/corruption_corpus), so downstream consumers
  that invoke it by path are unaffected; the build now just needs
  -p corruption-corpus --features driver.

Ten distortion families

channel · block · edge · noise · tone · overlay · chroma-boundary · aliasing ·
geometric · composite — each parameterized by region size (whole-image →
1/4 → 1/16 → 64×64 → 16×16 → 8×8, + 1px for edge/geometric) and severity
(opaque → 50% → 20% opacity). The subtle end (small region, low opacity) is the
hard case where a metric saturates and lets a real bug pass.

Design

• Deterministic — seeded from (ref_id, seed, params) via a dependency-free
  SplitMix64; same bytes on every platform. No rand, no OS entropy.
• Pure RGB-buffer math — applying a corruption needs no image decode, so the
  generators stay in the default build and compile to WASM. #![forbid(unsafe_code)].
• No corrupted images committed — the corpus is reproduced on demand from the
  curated reference set already in this repo (CLIC2025, CID22, KADID-10k, GB82,
  GB82-SC, …).
• driver feature (optional image dep) builds the score-ready quad
  (reference, corruption, q20-anchor, q10-anchor); without it, only the seeded
  in-memory generators + manifest types are compiled.
• Manifest ManifestEntry { ref_id, content_class, family, params, seed, expected_below_lq, expected_negative, source } — source reserves a
  real_bug slot for historical-bug reproductions tracked separately.

Files

• crate/corruption-corpus/src/{lib,families,prng,driver}.rs — crate root, 10
  families, seeded PRNG, quad driver (moved via git mv, history preserved)
• crate/corruption-corpus/examples/corruption_corpus.rs — emits quads +
  _MANIFEST.json on demand
• crate/corruption-corpus/tests/corruption_corpus.rs — driver integration tests
• crate/corruption-corpus/{Cargo.toml,README.md,CHANGELOG.md,.gitignore}
• crate/Cargo.toml — new [workspace] members; dropped the image dep,
  driver feature, and [[example]]
• crate/src/lib.rs — dropped pub mod corruptions;
• crate/{README,CHANGELOG}.md — point at the new crate

Tests (measured on this branch)

• cargo test -p corruption-corpus (default): 25 unit + 1 doc green
• cargo test -p corruption-corpus --features driver: 28 unit + 5 integration + 1 doc green
• cargo test -p codec-corpus: 26 unit + 4 integration (10 network-ignored) + 8 doc green
• cargo clippy --workspace --all-targets -- -D warnings (default + --features driver): clean
• cargo fmt --check: clean

Notes

• Real-bug reproductions (zenjpeg Huffman / chroma-boundary / partial-MCU / alpha
  bugs) are tracked in a parallel GitHub issue; this PR builds the synthetic
  families and leaves the real_bug source slot for them.
• The previously-noted clippy::unnecessary_map_or lint in tests/integration.rs
  is now fixed (is_some_and) in its own commit, so the workspace is
  clippy-clean under -D warnings.
• CI (.github/workflows/ci.yml) now runs the native + wasm jobs with
  --workspace (plus a -p corruption-corpus --features driver step), so the
  new member is exercised on every platform instead of being silently untested
  after the split.
• This branch also carries a few pre-existing imazen-26 corpus-data commits
  (synth pipeline + dimension/manifest/doc corrections) that were committed
  locally on the branch before the extraction; they are independent of the
  corruption-corpus change.