docs(render): S4-c2 contract (packet §8) — two FIFO alpha lists, the DrawMesh router, four flush sites, AM/FL transcript gate

Drafted from the five alphadepth captures (10,556 AM lines, every clip=0;
FL by return address 13,705 / 309 / 18 / 23) and the OH1 alpha-list
contract. Dispatch waits for S4-c1 to land.

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

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

@@ -327,3 +327,114 @@ the oracle for that pose).
   `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.
+
+## 8. S4-c2 contract (lead draft, 2026-09-03 night — dispatch after S4-c1 lands on the campaign branch; re-locate every owner at dispatch)
+
+**Scope (ONE production behavior change):** replace the single distance-sorted
+`RetailAlphaQueue` with retail's two append-order lists, route every delayed
+subset by the exact `DrawMesh` branch table, and flush at retail's four
+normal-world sites under retail's threshold rule. No evidence infrastructure
+beyond the transcript gate this chunk consumes (G-c2 below).
+
+**Capture profile the contract rests on (the five `*.alphadepth.log` files,
+counted 2026-09-03):** 10,556 `AM` lines, every one `clip=0` (so retail's
+`MultiPassAlpha` row 2 never fired live — default false, spec §3); `listSel`
+0 = 6,330 lines, 1 = 4,183 (two lists, both live); `FL` lines by return
+address: `005a1a0c` (DrawBlock 0.75) 13,705, `0059f310` (DrawBuilding 0f)
+309, `005a4877` (PView::DrawCells 0f) 18, `00453b90` (RenderNormalMode 0f)
+23; `new=1` 9,685 / `new=0` 871 (the first-for-list flag is mostly set —
+most DrawMesh invocations append one subset per list).
+
+**Retail facts the implementer verifies at the decomp BEFORE writing (quote
+the predicate, never paraphrase — `feedback_quote_decomp_predicates`):**
+
+1. `D3DPolyRender::AddMeshToAlphaList` @0x0059C230 — append only, two
+   static arrays of 3000 entries and two uint16 counts, returns false at
+   capacity (spec §5). Identify which `listSel` value is CLIP and which is
+   ALPHA from the bytes (the capture's `listSel` is the raw byte at
+   `@esp+0x18`), and record it in the code comment.
+2. `D3DPolyRender::FlushAlphaList(threshold)` @0x0059D2E0 — the early
+   return fires only when BOTH counts are under `threshold × 3000`; quote
+   the comparison (strict `<` or `<=`) from the bytes and pin the boundary
+   count the bytes give (spec §5 says equality at 2250 drains); drain CLIP
+   fully, then ALPHA fully, in append order; reset both counts; restore the
+   object matrix, not the material.
+3. `DrawMesh` branch table = spec §4 rows 1–5 verbatim; delay mask
+   `s_AlphaDelayMask` @0x00820D88 default `0x0E`; `MultiPassAlpha`
+   @0x0081EF96 default false; the constructed subset mask = spec §2
+   (surface type bits `0x00010300` → `0x02`, `0x04` → `0x08`, `0x10` →
+   `0x04`, else `0x00`; OR `0x01` when the polygon's signed stippling byte
+   is > 0).
+4. The four normal-world flush sites (spec §7) — confirm each capture
+   return address above lies inside the named function in
+   `docs/research/named-retail/symbols.json`.
+
+**C1 — two lists.** `RetailAlphaQueue` becomes two FIFO records, CLIP and
+ALPHA, capacity 3000 each. Entry = (source, token, list, overrideClipmap,
+first-for-list). NO distance field; `Submit` takes the list and the
+override flag, never a viewer distance. Append at capacity returns false
+and the subset is DROPPED (no immediate fallback) — pinned. Keep the
+`IRetailAlphaDrawSource` protocol (`PrepareAlphaDraws` once per drain with
+the tokens in list order, CLIP then ALPHA; only adjacent same-source
+entries form a batch — the "never group across another entry" invariant
+stays, it is what keeps compositing exact).
+
+**C2 — flush semantics and sites.** `Flush(threshold)` implements fact 2.
+Sites: `AlphaBarrier` (DrawBuilding) → `0f`; NEW leaf event at the
+sort-cell EXIT — its own `WalkFrameEventKind`, recorded after
+`OnLandscapeCellTurn` (the object turn) and before the next cell's land
+turn (packet §5 SETTLED) → `0.75f`; `LandscapeFlush` (PView::DrawCells) →
+`0f`; pass end (RenderNormalMode) → `0f`. `FlushFartherThan` deleted;
+AP-34 retired in the same commit; every `viewerDistance` argument on the
+submit path and `RetailAlphaOrdering.ComputeViewerDistance` (with its
+test) deleted — chunk 3's deletion list shrinks accordingly.
+
+**C3 — routing.** Both `WbDrawDispatcher` submit sites
+(`SubmitWalkAlphaInstance`, `DeferTransparentGroups`) and
+`ParticleRenderer`'s site route through ONE pure router implementing spec
+§4 with these inputs: currently-drawing-sky (true only inside the `Sky`
+leaf — weather emitters draw immediately there; `*.parts.log` is the
+oracle for which emitters ever appear immediate), the delay mask (one
+owner, default `0x0E`, no environment read), detail-surface presence (the
+active detail owner from VisualMaster), `MultiPassAlpha` (one owner,
+false), the spec §2 constructed mask from the DAT surface type and the
+polygon's stippling, and the material's `has_alpha`. Row 1 draws never
+touch a list. Truth-table test over every (mask `0x00`–`0x09`) × delay
+`0x0E` × sky × detail × multipass × has_alpha cell in spec §4, each row
+with a mutation text.
+
+**C4 — transparent EnvCell subsets** go through the same router (spec §6:
+the environment detail surface makes eligible subsets immediate).
+
+**C5 — register.** AP-34 deleted; any row describing the distance sort
+deleted; retail's non-restored material after a flush is a no-op under
+Vulkan's per-batch material binding — a code comment at the drain, not a
+row, unless the implementer can argue a visible effect.
+
+**Must NOT:** keep any distance anywhere on the alpha path; "recover" a
+capacity overflow; touch S4-c1's punch/seal machinery; add flush sites
+beyond the four; read an environment variable for the delay mask.
+
+**Tests (each with a mutation text showing the pin fails without the
+change — `feedback_every_new_pin_must_be_shown_to_fail`):** FIFO beats
+reversed distance in one cell; equal-key stable tie; two cells where a
+global sort disagrees with traversal; particle/object/transparent-cell
+overlap; DrawBuilding `0f` flush; pre-clear and final flush; the `0.75`
+valve exactly at the boundary count from fact 2 (and one below it: no-op);
+alternating blend modes across Vulkan batches; capacity 3000 overflow
+drops the subset; the router truth table; the new event kind's ordering.
+
+**Gate G-c2 (the chunk's real proof):** extend S4-c1's automated alphadepth
+transcript gate from PM/PC to AM/FL — per capture frame, the sequence of
+(flush site, threshold) and, per flush, the drained entry count PER LIST
+must match the capture's AM/FL stream; a diverging pose is tagged
+KnownFailure with a written explanation, never weakened. Then the walk/
+depth/alpha suites, Release build, the validation self-gate at the S3-closed
+poses diffed against the S3-closed run, and a stationary-phase soak A/B
+against the S4-c1 binary (removing the sort must not regress; #463's
+turn-phase rule).
+
+**Lenses:** one Sonnet implementer; two sequential review lenses on the
+session model (retail-faithful — every predicate quoted; production —
+allocation, the scratch budget, the gate's honesty); each lens returns
+PASS/FAIL with file:line findings; two fix rounds maximum, then the ledger.