feat: add sky backdrops with cloud decks, storms, and lightning

[solomonneas]

Adds a fourth backdrop family alongside the gradients, deep-space scenes, and geometric
patterns. A sky palette carries the colours and a SkyKind carries the structure, so
--palette and --sky compose the same way --palette and --scene already do.

Ten conditions: blue-hour, golden-hour, overcast, cirrus, mackerel, mammatus,
storm, bolt, aurora, crepuscular. Seven palettes sampled from photographs of the
named conditions, the same way the space palettes are sampled from astrophotography.

Everything is drawn from the style seed at render time on image + rand. No new
dependencies, nothing shipped as a bitmap, so a sky resolves at any card size and
--style-seed reproduces one exactly.

Existing seeds are unaffected

Sky palettes stay out of the default random rotation, which filters to
BackdropKind::Space (guarded by random_rotation_only_picks_space_palettes). They are
reachable by explicit --palette or by naming them in a pool, matching how the gradient
and pattern families already behave. Palette selection uses rendezvous hashing over names,
so a new palette only takes the seeds it wins and leaves the rest of the table alone.

Notes for review

• Every real defect in this work was invisible to the test suite. The full suite passed
  green while three of the ten scenes rendered as speckle, halftone dots, and rounded
  squares. They were only caught by rendering PNGs and looking at them. Structural tests
  (opaque everywhere, varies across the canvas, reproducible from a seed, kinds differ from
  each other) cannot catch this class of bug, so the added tests are a floor, not a
  guarantee.

• Preview any scene without needing a capture on disk:

  cloche studio --port 8137
  curl "http://127.0.0.1:8137/api/backdrop?palette=blue-hour&sky=bolt&seed=7&w=440&h=300" -o out.png
  

• ConfigCommand::Set is boxed because the args struct now carries one field per styling
  axis and tripped clippy::large_enum_variant. No #[allow] was added anywhere.

• storm is the weakest of the ten. Without bolts it reads as a murky field. It may not
  earn its place as a scene separate from bolt; happy to drop it if you agree.

Follow-on work for terrain backdrops (forest, ocean, dunes, mountains, plateaus) is
tracked in #41.

Verification

./scripts/verify
test result: ok. 212 passed; 0 failed

cargo fmt --check, cargo clippy --all-targets -- -D warnings, and cargo test, all
green.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch t3code/add-cloud-sky-backdrops

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