Document independent widget bounds and measured layout costs

docs/PLAN.md

@@ -872,7 +872,12 @@ is its *placement*, and the length a fraction resolves against is its *rel
 base*. A container says all three in one `PlaceDesc` argument. `Widget::draw`
 remains the only layout body. PR #19 also has optional size-request discovery
 for deferred min/max/clamp expressions (2026-09-20); ordinary widgets still
-return `Size`, and plain requests retain symbolic allocation. The invariants,
+return `Size`, and plain requests retain symbolic allocation. Built-in
+min/max/clamp rules also constrain the offered box; the dimension helpers
+edit the widget's own attributes, replacing the `MaxSize` wrapper. Preferred
+lengths and bounds are independent, so setting `.width` preserves `.max_width`
+and vice versa. Bound choices are retained as resolved lengths so moving a
+drawing cannot resolve them against a different box. The invariants,
 the measured costs, the fuzzing method and the failed hypotheses are all in
 `docs/LAYOUT.md`; the app's pin at
 `32f6ad8` predates every part of it, so nothing here depends on it until the