Record the chain-depth measurement and the question it opens

docs/IRIS_EXTRACTION_HANDOFF.md

@@ -25,6 +25,23 @@ The invariant the chain rests on: `resolved = region + resolve(slot)`, so
 anything that rewrites a region owes that slot a zero. `mov` does it for the
 subtree it rewrites and `draw_inner` for the widget it draws.
 
+**Waiting on the owner, and measured first.** `tests/chain_cost.rs` times the
+pass on the GPU with timestamp queries: at 200,000 instances the walk is free
+to about depth 8 (+5%) and then costs roughly 3 us per level -- +43% at 16,
++221% at 64. Each step is a storage load addressed by the previous one, so it
+is the chaining that costs and not the arithmetic at a level; a slot carrying
+a whole region would measure the same. Since #18 gives every active widget a
+slot, a primitive resolves through its full tree depth, which LAYOUT.md notes
+has exceeded 16. Irrelevant at an example's couple of hundred primitives,
+squarely in that regime for a transcript's glyphs.
+
+So the open question is how to keep the chain shallow -- a slot only where
+something is actually moved (needs re-parenting when an intermediate ancestor
+later gains one), against §6's rejected flattening, which the number partly
+rehabilitates. Retiring `Remap` by way of region chaining is cheap on top of a
+shallow chain and expensive without one. Put to her on #18; do not act before
+she picks.
+
 Check for a review before starting anything, and read the newest
 `submitted_at` rather than the first result: