docs(render): S4 packet — settle the valve hook and the alpha capture format questions

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

docs/research/2026-09-01-overhaul/s4-depth-alpha-packet.md

@@ -126,12 +126,19 @@ seal/punch color-state row from S4-c1 kept or retired per its decision.
 
 ## 5. Open questions to settle before dispatch
 
-- Does the walk's `SC` turn (per land cell) have a leaf hook where the 0.75
-  valve belongs, or does `OnSortCellTurn` need a third leaf method? (Read
-  `WalkFrameDriver`'s land-cell turn after chunk 3's interleave.)
+- SETTLED (2026-09-03): the walk's `SC` turn is `IWalkEventSink.OnSortCellTurn`
+  in `WalkFrameDriver` (today a transcript print at `DrawSortCell` ENTRY,
+  fired from `RetailFrameWalk.DrawLandscape`); the 0.75 valve is retail's
+  `DrawBlock` call AFTER `DrawSortCell` returns, so chunk 2 adds one leaf
+  method at the sort-cell EXIT (a `WalkFrameEventKind` of its own, ordered
+  after the cell's object turn) — not a print-site change.
 - Where does the particle submitter's list membership come from — the
   particle draw is retail's own `AddMeshToAlphaList` consumer (billboards go
   to ALPHA), or is it drawn through the emitter's cell turn immediately?
   Check `ParticleRenderer.cs`'s submit site against the spec's §4 table.
-- The `alphadepth` capture format: confirm the `AM`/`FL` lines carry the list
-  id so the transcript can be compared per list, not merged.
+- SETTLED (2026-09-03): the `alphadepth` captures' `AM` lines carry the list
+  selector (`AM mesh=… surf=… csurf=… new=… clip=… listSel=…` — `listSel`
+  is the list, `new` the first-for-list flag, `clip` the clip-map duplicate,
+  `surf`/`csurf` the raw surface type and material), and `FL` lines mark
+  every flush (terrace-edge: 3,515 `FL` lines over its frames), so the
+  chunk-2 transcript compares per list and per flush site, not merged.