r2il-mask-abi-probe: DAV candidate filter through quack and R2IL on one evaluator (Round 4)

.claude/board/entries/2026-10-06-quack-r2il-one-evaluator-round4.md

@@ -0,0 +1,66 @@
+# 2026-10-06 — Quack and R2IL on one evaluator: the DAV candidate filter (Round 4)
+
+## MEASURED
+
+`crates/r2il-mask-abi-probe/tests/row_bridge.rs`, § "Round 4" (4 new tests;
+the census test extended). `FoldDialect` gains four lowering arms. Each maps
+onto a `Pred`/`MaskOp`/`Terminal` mask-risc already had, and none adds a
+kernel:
+
+- `LOAD` space 3: a resident mask plane as an `Addr::Plane`, used as
+  `Operand::Plane`
+- `IntNot` (R2IL ordinal 23) → `MaskOp::Not`
+- `RANGE` (fold band, arity 0, bounds as immediates) → `Pred::Range`
+- `KEEP` → `Terminal::Keep` into a caller-lent sink
+
+The DAV candidate filter `is_null(valid) AND lo <= row < hi` is spelled
+three ways: `quack::lower`, R2IL bytes NOT-first, R2IL bytes RANGE-first.
+The kept mask is bit-identical to a row oracle at n ∈ {21, 64, 65, 130,
+256, 1000}, and so is the count. All paths execute through
+`mask_risc::execute_into`.
+
+Physical work at n = 1000 (16 words):
+
+| path | passes | derived words | lowering |
+|---|---|---|---|
+| `quack::lower` | 3 (`Not`, gated `Range`, trailing `And`) | 48 | Tiled |
+| R2IL NOT-first | 2 (`Not`, `Range` gated under it) | 32 | Tiled |
+| R2IL RANGE-first | 3 (`Range`, `Not`, `And`) | 48 | Tiled |
+
+The COUNT forms are all `Tiled` too. Positive-polarity control
+(`plane AND range`, COUNT): quack and R2IL both lower to ONE `Range` gated
+under the resident plane, which becomes `Lowering::Range`. That path writes
+no derived words, and both match the oracle.
+
+KEEP-path dialect allocation is 16,648 B at both 16,384 and 1,048,576 rows.
+
+Disable runs, red: the plane-AND peephole replaced by a real `And`;
+`IntNot` without the complement; `RANGE` losing its upper bound; `KEEP`
+writing nowhere; `KEEP` pushing a status scalar; a population-sized buffer
+allocated per fold.
+
+The peephole was made commutative to fold the RANGE-first order. mask-risc
+refused the result with `ScratchReadBeforeWrite`: an earlier pass would be
+gated under a later slot.
+
+## FINDING
+
+- **One evaluator holds.** The DAV filter needs no second population
+  evaluator, only lowering arms in the R2IL dialect.
+- **Pass count is byte-order dependent on the R2IL side; quack's is not.**
+  Folding the RANGE-first order needs the `Not` hoisted ahead of the
+  `Range` (a scheduling rewrite), not a wider peephole.
+- **The zero-write fold is blocked by polarity, in both frontends.**
+  `fused_terminal` folds a `Range` gated under a resident plane. The DAV
+  candidates need the complement of that plane, which exists only once
+  written into a scratch slot. So the missing algebra is narrow: a
+  `Range` gate with negative plane polarity (or a resident "unobserved"
+  plane).
+- **The shared middle is Address / mask expression / fold / Scalar /
+  Terminal.** The dialect's stack holds only `Scalar`, `Slot` and
+  `Address` (lane, `VIA`, plane).
+
+## OPEN
+
+- quack's trailing `And` after a gated conjunct is now pinned in two
+  programs (the join and the DAV filter).

.claude/board/entries/README.md

@@ -25,11 +25,12 @@ index row, (3) no duplicate entry id. Checks 1 and 2 are deliberately
 opposite directions; the stranding this convention prevents shows up in
 exactly one of them, never both.
 
-223 entries, 2026-08-06 .. 2026-10-06.
+224 entries, 2026-08-06 .. 2026-10-06.
 
 | date | entry id | finding | file |
 |---|---|---|---|
 | 2026-10-06 | `v3-v4-dual-reading-round3` |  | [2026-10-06-v3-v4-dual-reading-round3.md](2026-10-06-v3-v4-dual-reading-round3.md) |
+| 2026-10-06 | `quack-r2il-one-evaluator-round4` |  | [2026-10-06-quack-r2il-one-evaluator-round4.md](2026-10-06-quack-r2il-one-evaluator-round4.md) |
 | 2026-10-06 | `D-MORTON-0` |  | [2026-10-06-morton8x8-checked-address.md](2026-10-06-morton8x8-checked-address.md) |
 | 2026-10-06 | `dav-resident-reading-round2` |  | [2026-10-06-dav-resident-reading-round2.md](2026-10-06-dav-resident-reading-round2.md) |
 | 2026-10-06 | `dav-carrier-invariance-quack` |  | [2026-10-06-dav-carrier-invariance-quack.md](2026-10-06-dav-carrier-invariance-quack.md) |