fix(background): halo zones can be wide ellipses, not just circles

[LeadcodeDev]

Refs #344 — its point 2.

HaloZone had a single radius, so the halo animated-background preset could only draw circles. The reference video #344 was rebuilt against needs a thin, wide elliptical light along the top edge, and a rotated one. Neither could be expressed.

What changed

HaloZone gains radius_x, radius_y (both Option<f32>, falling back to radius) and rotation in degrees, plus effective_radius_x(), effective_radius_y() and is_circular().

Backward compatibility is the point, so it is tested byte-for-byte

Every scenario using plain radius must render identically, and the implementation earns that rather than assuming it: a circular zone takes the exact original code path — same expression order, same draw_circle call — instead of being routed through draw_oval and trusting Skia to match drawCircle bit-for-bit on anti-aliased edges.

radius_only_zone_renders_byte_identically_to_explicit_equal_radius_x_radius_y pins it.

is_circular() deliberately ignores rotation: rotating a true circle is a no-op, and going through a rotation matrix for zero visual gain would perturb anti-aliased edge pixels.

Two details worth knowing

Blur follows the short axis. A non-circular zone sizes its blur off min(radius_x, radius_y), so a thin wide band does not get a blur scaled to its length and wash out.

The transition interpolation was fixed too. lerp_zones — the a_zones/b_zones path used by interpolate_animated_bg — now interpolates the three new fields, so a halo-to-halo scene transition will not snap the shape. That path is currently #[allow(dead_code)] and not yet wired to a caller; it was fixed anyway because it is the obvious future wiring point and a half-updated interpolator is the kind of thing that is found much later, from a render.

Verification

14 tests: 7 on the schema helpers (defaults, fallback, the is_circular() truth table including the rotation case), 5 on the painter, 2 on the transition interpolation.

Proven to catch the absence — reverting draw_bg_halo to the circle-only body, with the schema fields present but unused:

a_wide_ellipse_reaches_further_horizontally_than_vertically panicked:
  a radius_x=0.45 ellipse should still be near full brightness 80px from center

rotating_the_ellipse_changes_which_pixels_it_covers panicked:
  assertion `left != right` failed: a 90-degree rotation on a non-circular
  zone must change the render

cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace (1540) all clean.

Note

The three new fields are snake_case (radius_x, radius_y), unlike their kebab-case neighbours world-position and animated-background. That inconsistency is the schema's, not a typo — rules/halo-shapes.md says so explicitly so a generator copies the casing rather than guessing it.

Written comment-free, per the codebase-wide rule from #345.