Add exact color-to-variable suggestions and fixes

[lmn451]

Summary

• add exact normalized matching from literal colors to workspace CSS custom properties
• surface matches as replacement completions, informational diagnostics, and quick-fix code actions
• collect literal colors across supported CSS parsing contexts, including compound values while skipping existing var(...) usages

Implementation details

• add colorVariableFeature to centralize diagnostic, completion, and code-action generation
• extend CssVariableManager to index literal color ranges and resolve variables by normalized color
• extend color parsing to support both modern space-separated color syntax and legacy comma-based alpha forms
• advertise code action support and add completion trigger characters for literal color editing

Tests

• add focused coverage for normalized matching, diagnostics, completions, quick fixes, and multi-context parsing
• run npm test
• run npm run compile

Notes

• bd is not installed in the execution environment, so repo-local issue state could not be updated here
• Linear issue reference is still needed if you want the PR description to comply fully with repo policy