Codebase audit improvements + programmatic before/after previews for all operations

[subwaymatch]

Summary

This PR delivers a codebase audit with fixes, and restores the old README's before/after sample images — now generated programmatically for every operation.

Audit improvements

• Removed the matplotlib dependency. It was installed (~40 MB with transitive deps) for exactly two functions. layeris.color.hsv now provides pure-NumPy rgb_to_hsv/hsv_to_rgb that are bit-identical to matplotlib's implementations (verified on random and adversarial inputs). Runtime deps are now just NumPy, Pillow, Requests.
• from_url hardening: request timeout + raise_for_status(), so a 404 raises a clear HTTPError instead of a cryptic image-decoding failure.
• EXIF orientation is applied on load, so phone/camera photos open upright.
• Input validation with actionable errors: constructor rejects non-float dtypes and bad shapes; blend opacity must be in [0, 1]; curve validates channel letters and control-point count; resize rejects non-positive sizes; malformed hex colours raise ValueError (and #rgb shorthand is now supported).
• Packaging fixes: added the [dev] extra that the README referenced but didn't exist; refreshed requirements.txt (was pinned to 2019 versions inconsistent with pyproject.toml); replaced the deprecated Image.LANCZOS alias; version bumped to 0.4.0.
• CI: GitHub Actions workflow running the test matrix on Python 3.10–3.13 plus a preview-generation smoke test.
• Tests: suite grew from 141 to 187, covering the HSV port (known values, roundtrip, hue wrap), hex parsing, and all new validation paths.

Programmatic before/after previews

• scripts/generate_previews.py regenerates the whole gallery deterministically with python scripts/generate_previews.py. A single operations table drives:
  • docs/previews/*.jpg — 24 side-by-side before/after composites (left = input, right = result), one per operation: 9 adjustments, all 14 blend modes, and a chained pipeline;
  • docs/index.html — a static gallery page, ready for GitHub Pages (enable Settings → Pages → Deploy from a branch → /docs);
  • the README gallery markdown (--markdown), so snippets, descriptions, and images can't drift apart.
• Previews reuse the original demo photo and the old README's blend colours; darken-group modes are applied to a grayscale base, mirroring the original sample images.
• README now shows every operation with its description, snippet, and before/after image, plus regeneration instructions.

Test plan

• pytest — 187 passed (run with matplotlib uninstalled to prove the dependency is gone)
• HSV port verified bit-identical to matplotlib.colors on random arrays, primaries, grays, and boundary values
• python scripts/generate_previews.py run twice — byte-identical output (deterministic)
• docs/index.html rendered in headless Chromium and visually inspected
• CI workflow added (will run on this PR's branch after merge to main; pull_request events run it immediately)

🤖 Generated with Claude Code

https://claude.ai/code/session_01KHVMC2EHFz4h5oiFTuD2Hs

---

Generated by Claude Code