fix(css): parse all common colour and length forms, fail loudly on the rest

[LeadcodeDev]

Part of #102 — wave 1.

Mini-spec

Goal: accept every colour and length form an author would reasonably write, and fail loudly on the rest instead of silently rendering black or zero.

Invariants

• No colour string ever resolves to an unintended SColor::BLACK. Either it parses, or it is surfaced as an error.
• No length string ever silently resolves to 0px.
• parse_hex_color keeps its current infallible signature so existing call sites are untouched.

Frozen contract (from #102) — this workstream owns and must expose exactly:

pub fn parse_css_color(input: &str) -> Option<(u8, u8, u8, u8)>

Out of scope: wiring a validation-time error for unresolved colours (L4 consumes this API), gradients.

Acceptance

• #fff, #FFF, #ffff, #ffffff, #ffffffff all parse.
• rgb(255,255,255), rgba(255,255,255,0.5), hsl(...), hsla(...) parse, with and without spaces.
• The CSS named-colour set is supported (currently 11 names are hardcoded at paint_pass.rs:1338-1352).
• An unresolvable colour is reported, not silently blackened.
• units.rs:139-141 / 163-165 no longer fold an unparseable length into Px(0.0) without a signal.
• Unit tests cover each accepted form and at least three rejected ones.

Files: crates/rustmotion-core/src/engine/renderer/colors.rs, crates/rustmotion-core/src/engine/paint_pass.rs, crates/rustmotion-core/src/css/units.rs

Verify

rtk cargo test -p rustmotion-core
rtk cargo build --release

[LeadcodeDev]

Merged into main via #108 (squash 38596ca).