Fix fill text MeasureFunc constraint, strengthen nested flex prompt

layout.ts: When text fills width (stretch in col parent), MeasureFunc
now uses Yoga's constraint width directly instead of clamping to
the 100px default fixedWidth.

prompt: Emphasize that ALL intermediate flex containers need w="fill",
not just the first level. grow inside HUG = zero width.

packages/core/src/layout.ts

@@ -427,8 +427,9 @@ function configureTextLeaf(
       yogaChild.setWidth(fixedWidth)
     }
     yogaChild.setMeasureFunc((width, widthMode, _height, _heightMode) => {
-      const constraintW =
-        widthMode === MeasureMode.Undefined ? fixedWidth : Math.min(width, fixedWidth || width)
+      const constraintW = fillsWidth
+        ? (widthMode === MeasureMode.Undefined ? fixedWidth : width)
+        : (widthMode === MeasureMode.Undefined ? fixedWidth : Math.min(width, fixedWidth || width))
       const cacheKey = Math.round(constraintW)
       const cached = cache.get(cacheKey)
       if (cached) return cached

src/ai/system-prompt.md

@@ -34,7 +34,7 @@ justify/items require flex — always declare flex="row" or flex="col" before us
 
 A hug parent shrinks to fit its children. A fill child stretches to its parent. These can't be circular — if the parent hugs, at least one child must have a concrete size.
 
-Nested flex containers inside a column parent need w="fill" to stretch, otherwise they collapse to zero width.
+Nested flex containers inside a column parent need w="fill" to stretch, otherwise they collapse to zero width. ⚠ This applies to ALL intermediate containers, not just the first level. If you have `flex="col" > flex="col" > flex="row"`, EVERY level needs w="fill". A `grow={1}` child inside a HUG parent gets zero width — the parent must have a concrete size (fixed w or w="fill").
 
 There is no margin property. Padding on a container pushes all children equally from the edges. To offset a single child, wrap it in a Frame with its own padding.