iris: make span compaction explicit

docs/PLAN.md

@@ -847,6 +847,15 @@ such as corner radius and text's position within horizontal overflow use
 `Len`. Flexible padding participates in the same proportional allocation as a
 span rather than silently discarding its `rest` component.
 
+**A `Span` leaves its children in the offered orthogonal region unless
+explicitly compacted** (2026-09-12). It still reports the widest orthogonal
+child as its intrinsic size, so a row nested in a column keeps its content
+height rather than claiming the column's remaining height. By default it does
+not squeeze child regions to that intrinsic size, because doing so makes child
+alignment operate inside the widest child rather than a known-width container.
+`.compact()` opts into that shrink-to-widest-child placement for deliberately
+dense groups.
+
 **Text overflow is treatment plus position, not alignment** (2026-09-12).
 `TextOverflow` selects `Visible`, `Wrap`, `Hidden`, or the single `Ellipsis`
 treatment. Unwrapped hidden and ellipsized text retains one canonical shaped