Arbitrary value type inference for polymorphic utilities

[goulvenclech]

Some Tailwind CSS v4 utility prefixes are polymorphic — the CSS property emitted depends on the value type inside brackets:

• text-[#f00] → color: #f00 (colour)
• text-[14px] → font-size: 14px (length)
• bg-[#f00] → background-color: #f00 (colour)
• bg-[url(...)] → background-image: url(...) (url)

The engine needs to:

1. Detect value type from raw bracket content (colour, length, percentage, url, etc.)
2. Dispatch to the correct CSS property based on inferred type
3. Support explicit type hints as override: bg-[color:var(--my-bg)], text-[length:var(--size)]

Depends on #20.