Replace placement calls with region nodes

tests/replace_cost.rs

@@ -1,4 +1,4 @@
-//! What re-placing a subtree costs per frame, as a load for a counter rather
+//! What remapping a subtree costs per frame, as a load for a counter rather
 //! than a check. A span of 200 fixed-height rows, five primitives each, with
 //! the row above them changing height every frame, so every row below is
 //! offered a box the same shape somewhere else.
@@ -6,9 +6,7 @@
 //!     cargo test --release --test replace_cost -- --ignored
 //!     perf stat -e instructions:u target/release/.../replace_cost-* --ignored
 //!
-//! Wall time is the wrong number here; see `draw_cost.rs`. Measured on
-//! 2026-09-14 at 1.98M instructions per frame, against 2.38M for rewriting
-//! each row's regions instead and 7.13M for redrawing them.
+//! Wall time is the wrong number here; see `draw_cost.rs`.
 
 use iris::harness::Harness;
 use iris::prelude::*;
@@ -18,7 +16,7 @@ const FRAMES: usize = 200;
 
 #[test]
 #[ignore = "measurement, not a check"]
-fn replacing_rows_every_frame() {
+fn remapping_rows_every_frame() {
     let mut h = Harness::new((1920, 1200));
     let first = rect(Color::RED).height(40).add(&mut h.rsc);
     let mut span = Span::empty(Dir::DOWN);