iris: count text layouts, so "a delta shapes one block" is measured rather than argued

take_counters gains a fourth counter, text shapes, bumped in
Painter::render_text -- which TextView::render only reaches on a cache
miss, so it counts shapes and not requests. A draw counter cannot stand
in for it in either direction: a widget can be redrawn without
re-shaping (the layout is memoized by width) and re-shaped without any
extra draw, and re-shaping is the whole thing the per-block transcript
row exists to avoid.

With it, a_delta_into_a_long_reply_redraws_the_same_widgets_as_a_short_one
asserts the number docs/DECISIONS.md's 2026-09-06 entry actually claims:
one delta into a 100-paragraph reply shapes exactly one text layout, the
same as into a one-paragraph one. Before the split that was necessarily
O(message), since the reply was one buffer.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

iris/core/src/ui/painter.rs

@@ -191,6 +191,10 @@ impl<'a> Painter<'a> {
         width: Option<f32>,
     ) -> RenderedText {
         let density = self.state.density;
+        // Counted here rather than in `TextView::render`, which returns
+        // its memoized layout without reaching this -- so this counts
+        // shapes, not requests. `UiRenderState::take_counters`.
+        self.state.shape_count += 1;
         let ui = self.rsc.ui_mut();
         ui.text
             .render(buffer, attrs, width, &mut ui.textures, density)

iris/core/src/ui/render_state.rs

@@ -55,6 +55,9 @@ pub struct UiRenderState {
     draw_count: u64,
     region_mut_count: u64,
     mov_count: u64,
+    /// Text layouts actually computed -- bumped by `Painter::render_text`,
+    /// which `TextView::render` only reaches on a cache miss.
+    pub(super) shape_count: u64,
 }
 
 /// A move chain more than this deep would mean something else is wrong
@@ -76,17 +79,25 @@ impl UiRenderState {
             draw_count: 0,
             region_mut_count: 0,
             mov_count: 0,
+            shape_count: 0,
         }
     }
 
     /// Reads and zeroes the (draws, region_mut rewrites, move_offsets
-    /// writes) counters -- call once per frame before `update()` to
-    /// measure exactly that frame, per LAYOUT.md section 8.
-    pub fn take_counters(&mut self) -> (u64, u64, u64) {
+    /// writes, text shapes) counters -- call once per frame before
+    /// `update()` to measure exactly that frame, per LAYOUT.md section 8.
+    ///
+    /// The fourth is the one a draw count cannot stand in for: a widget
+    /// can be redrawn without re-shaping (`TextView::render` memoizes by
+    /// width) and re-shaped without any extra draw, and it is re-shaping
+    /// that the per-block transcript row exists to avoid -- see
+    /// `transcript_ui`'s `a_delta_into_a_long_reply_shapes_one_block`.
+    pub fn take_counters(&mut self) -> (u64, u64, u64, u64) {
         (
             std::mem::take(&mut self.draw_count),
             std::mem::take(&mut self.region_mut_count),
             std::mem::take(&mut self.mov_count),
+            std::mem::take(&mut self.shape_count),
         )
     }