Improve color parser: support short hex #RGB/#RGBA and named colors

[ZhukMax]

Extend EFx color parsing to accept the short hex forms #RGB and #RGBA (nibble-doubling) in addition to existing #RRGGBB and #RRGGBBAA. Add support for a curated set of named colors (e.g., white, black, red, tomato). Provide precise diagnostics for invalid lengths, non-hex input, and unknown names.

• Developer/designer friendly: short hex is common in style guides and hand-written templates.
• Better compatibility with assets and snippets copied from design tools/docs.
• Improves DX by reducing verbosity and offering readable named aliases.

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Scope

• Accepted formats

  • #RGB → #RRGGBB via nibble doubling.
  • #RGBA → #RRGGBBAA via nibble doubling.
  • #RRGGBB, #RRGGBBAA (already supported).
  • Named colors: a small, well-chosen set (e.g., black, white, gray, red, green, blue, yellow, cyan, magenta, orange, purple, pink, tomato, teal, indigo, slate, emerald, …).

• Optional feature flag css-color-names to enable the full CSS list (~140 names).

• Output stays the same (egui::Color32 / ecolor::Rgba conversion inside renderers).

Non-goals (here):

• rgb()/rgba()/hsl() functional notation.
• Color space conversions beyond sRGB.

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Proposed API (sketch)

Parser surface

// efx-core
pub fn parse_color(value: &str) -> Result<ColorRgba, ParseError>; 
// extends existing logic (#RRGGBB[AA]) with #RGB/#RGBA + names

Template usage

efx!(ui, r#"
  <Label color="#abc">Short Hex</Label>
  <Label color="#abcd">Short Hex + Alpha</Label>
  <Label color="tomato">Named Color</Label>
"#);

Diagnostics

• Invalid hex length → “Expected #RGB/#RGBA/#RRGGBB/#RRGGBBAA; got …”
• Non-hex characters → show first offending index.
• Unknown name → list closest matches (e.g., “Did you mean tomato?”).

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Tasks

• Implement short-hex handling: detect #RGB/#RGBA, expand nibbles, parse to RGBA.

• Introduce a &'static name→RGBA map:

  • Minimal built-in set (hand-curated).
  • Optional css-color-names feature for the full CSS table.

• Integrate with existing attribute adapters (color, bg, etc.).

• Diagnostics:

  • Clear errors for bad length/non-hex.
  • Fuzzy suggestions for unknown color names.

• Tests:

  • Unit: nibble-doubling, alpha defaults, name lookups.
  • trybuild UI tests: invalid strings, unknown names (assert message text).

• Docs:

  • Update docs/tags.md with a “Color values” section (formats, examples).
  • Mention the css-color-names feature and list of guaranteed built-ins.

• Ensure examples build under wasm32-unknown-unknown (no runtime launch).