💡 Parent-relative min/max (percent) constraints for sizing

Start with the failing test in test/percent-sizing.test.ts and its nm/repro/percent-sizing diff, then read Clay_SizingMinMax in clay/clay.h:316-328. Trace how sizing bounds reach the layout engine and determine the API and wire changes needed. Done means parent-relative floors and caps preserve absolute-cell behavior and the test passes.

Is your feature request related to a problem?

There's no way to size an element relative to its parent as a constraint rather than a fixed size. We can say "exactly 50% of the parent" with percent(0.5), but not "at least 50%" or "at most 50%" while still letting content drive the actual size. A sidebar that should be ≥50% tall but grow with its content, or a panel capped at half the viewport, can't be expressed: percent(0.5) pins to exactly 6 rows in a 12-row parent regardless of content, and fit()/grow() only take absolute cell counts in their min/max slots. (Passing a percent into a min/max slot today corrupts layout—it's coerced into a garbage float.)

Describe the solution you'd like

A way to express parent-relative floors and caps, not just fixed sizes, so an element can be bounded by a fraction of its parent while content still determines the size within those bounds. The exact API is open; a few directions worth weighing:

• Percent in the existing min/max slots — fit(percent(0.5)) / fit(0, percent(0.5)). Smallest surface, but overloads the min/max args.
• Dedicated clamp helpers — clamp(min, max) or minmax(min, max) accepting cells or percent(...), composing with fit()/grow(). More explicit about intent.
• min() / max() constraint helpers — CSS-like, e.g. max(fit(), percent(0.5)). Most general, largest surface.

Whatever the shape: numeric bounds must keep exact absolute-cell semantics (additive only), and a percent bound should clamp content-aware—a floor still lets content grow past it, a cap still lets content shrink below it.

Describe alternatives you've considered

Measure the parent first (info.get(parent).bounds), compute the cell count, and feed an absolute number back on a second pass—but that needs two renders, goes stale on resize, and can't express floor-plus-content-grows in one pass. The whole-axis percent() is the existing primitive that falls short here.

Additional context

Failing test case on nm/repro/percent-sizing (test · diff). Whichever shape wins, the clamp lives deepest in the layout engine: Clay_SizingMinMax (clay/clay.h:316-328) is absolute float cells with no parent-relative bound, so it needs a percent-aware min/max on the wire and in the engine.