fix(world): size world backgrounds from the camera's extent, not viewport * 5

[LeadcodeDev]

Closes #189.

What

The same halo config behaved differently in a slide view and a world view: a radius: 0.55 that reads as a half-screen glow became a >5000px wash flooding every frame.

crates/rustmotion/src/engine/render/background.rs:68:

let world_w = width * 5.0;
let world_h = height * 5.0;

while HaloZone documents its fields as fractions of width/height.

Two problems in one line:

• The unit changed with the view type, silently, for the same field.
• 5.0 was unrelated to the world's actual extent, which is determined by the scenes' world-position values. A world spanning two viewports got the same canvas as one spanning ten — so calibrating a halo was trial and error with no rule to reason from.

How

WorldTimeline::world_extent(vw, vh) returns the union of one viewport-sized rect per camera waypoint — exactly the span the camera ever shows — as (x, y, width, height). The origin travels with it, so negative world-positions are inside the world rather than off its edge, which the old 0..w*5 framing could not express.

draw_world_bg_with_parallax takes that extent instead of computing its own, and HaloZone's doc comments name the surface for both view types.

Visual change

Any world-view halo calibrated against the old 5x canvas now covers a different share of the frame — proportionally larger for a world smaller than five screens, which is most of them. Checked against the 3-beat world view in the showcase scenario: the halos went from near-invisible points to a proportionate ambient glow, text still legible, register unchanged. Slide views are untouched.

Tests

Four on world_extent, no rendering needed:

• spans the waypoints plus one viewport
• a single-waypoint world is one screen — where viewport * 5.0 claimed five, and divided every radius by five with it
• negative waypoints move the origin rather than being measured from zero
• no waypoints falls back to the viewport

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