Run source measurement and arrangement in the frame pipeline

src/api.rs

@@ -770,6 +770,8 @@ declarations, explicit first `self` parameters, positional field construction an
 member assignment. Inherited methods bind the actual receiver; overridden methods
 are dynamically selected through self. Concrete native visual subclasses reuse real native allocators; abstract bases remain unconstructible. Traits and annotation checking remain pending. Native Block/Page methods use the same type method tables and binding path.
 
+Source measure and arrange overrides execute bottom-up and top-down in one read-only VM layout call. Structural Mount links exist before first measurement; bounds queries reject premature geometry. Native base layout uses current occurrence child measurements, and Arrangement must assign every stable child slot exactly once. Layout dependencies changed by lifecycle hooks trigger another pass before publication. Constraints are currently finite/unconstrained page inputs; viewport propagation and source measurement caching remain pending.
+
 Source paint overrides execute in the actual frame pipeline under a read-only heap policy. Source hitTest overrides similarly refine native pointer targets in VM paint order; Point.inside(bounds) and Point.inCircle(cx, cy, radius) provide primitive shape queries. Managed DisplayList/ObjectList/PaintCommand and Constraints/Bounds/Point results are ordinary runtime objects.
 
 Native input POST /events is a transport envelope, not Endor source. The window