Accept clamp() in length properties

[Atlas07]

Accept clamp() in length properties

clamp() was registered as a CSS function but never listed in the extended-length value rule — so width: clamp(1rem, 2vw, 3rem) failed to validate, even though min() / max() / calc() worked.

Fix

Added <clamp()> to extended-length and threaded the Function_clamp variant through both renderers and the runtime type, mirroring min()/max().
One shared rule → every length property gets it (width, height, padding,margin, font-size, gap, inset, …).

Scoped to extended-length, which covers essentially all real-world usage.
Percentages/angles/times can follow up. Relates to #413 / #427.

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[davesnx]

Perfect