feat(opstack): every op declares its channels -- ordering becomes derivable

The composer must assemble stacks nobody wrote by hand. Today the legal
order of operators lives in a human's head and in the hand-written
builders. COMPOSER-NOTES §3.2: an op's legal position is fixed by which
channel it READS and which it WRITES, so dependencies form a DAG and any
topological sort of that DAG is a legal stack.

EVoxelOpChannel names the two fields FVoxelOpSample actually carries
(Density, Sdf) -- read from the struct, not assumed. All 29 concrete
density ops declare reads, writes and additive/transformative, filled in
by reading each Eval rather than trusting its name. The declarations are
PURE VIRTUAL so a new op cannot silently omit them; the composer's safety
depends on every op answering.

A declaration nobody checks is a comment, so ValidateChannelOrder exists
and VoxelForge.OpStack.ChannelDAG runs all eight shipping stacks through
it. All pass: TunnelNetwork/Underwater 20 ops, Maze 8, VerticalShafts 9,
FloatingIslands 8, FlatPlain/CrystalChamber 6, SurfaceWorld 6.

Metadata is snapshotted in FVoxelOpStack::Add, so no declaration virtual
is ever called per voxel -- ARCHITECTURE §8.10 respected.

⚠️ THE FUSE WAS DELIBERATELY NOT APPLIED, and the reason is the finding I
asked for instead. §3.2 assumed FFloorBiasMod sits next to
FCaveRoughnessMod. It does not: TEN ops separate them (Terrace, LayerLine,
Ribbing, Overhang, Cliff, Scallop, Arch, RoomColumn, Dome, Pinch), and all
ten read and write Density. Moving FloorBias would reorder the live
density accumulation, and IEEE-754 addition is not bitwise commutative --
so the note's claim that fusing "leaves the field bit-identical" is false
as written.

That leaves a real open question for the composer, recorded not resolved:
channel rules alone say any Density->Density order is legal, but they
cannot express "this op CORRECTS that one". Either the fuse proceeds and
accepts a changed field (legitimate -- peer determinism never required
bit-identity across builds), or corrective intent needs its own
declaration. Settling it needs FFloorBiasMod's actual math read, which is
the next question I put to Codex.

Build clean, 22 succeeded / 0 failed, verified in VoxelM.log.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VJiTcTSDNkKBtnNdiBtdaU

COMPOSER-NOTES.md

@@ -88,12 +88,16 @@ terrain from those, deterministically, exactly as today. **No runtime compositio
 
 ### 3.2 Structure: how a stack gets assembled
 
-The three real builders, for reference:
+The current shipping builders, for reference (structural posts included; TunnelNetwork's builder is
+also the Underwater builder):
 
 ```
-TunnelNetwork (19)   rock source -> room graph (SDF) -> SdfCarve -> 12 density modifiers -> worm source -> structural
-Maze (7)             rock source -> lattice corridors (SDF) -> SdfRoughness (SDF) -> SdfCarve -> structural
-FloatingIslands (7)  VOID source -> island blobs (SDF) -> SdfRoughness (SDF) -> SdfFill -> structural
+TunnelNetwork / Underwater (20)  rock source -> room graph (SDF) -> SdfCarve -> 12 density modifiers -> worm source -> structural
+FlatPlain / CrystalChamber (6)   slab-void source -> grid columns -> structural
+Maze (8)                          rock source -> lattice corridors (SDF) -> SdfRoughness (SDF) -> SdfCarve -> structural
+SurfaceWorld (6)                  surface column -> overhang -> structural
+VerticalShafts (9)                rock source -> shaft field (SDF) -> SdfRoughness (SDF) -> SdfCarve -> ledges -> structural
+FloatingIslands (8)                VOID source -> island blobs (SDF) -> SdfRoughness (SDF) -> SdfFill -> structural
 ```
 
 **⭐ Most ordering is MECHANICAL — derive it, do not author it.** Roughness sits on the *SDF* before
@@ -103,6 +107,13 @@ which it writes**.
 ⇒ **Every operator declares its channel reads/writes.** Dependencies form a DAG and **any topological
 sort of that DAG is a legal stack** — no authored order, and it produces orderings nobody wrote down.
 
+**Built status — 2026-09-04:** `EVoxelOpChannel` now names the two fields actually carried by
+`FVoxelOpSample` (`Density` and `Sdf`); all 29 concrete density operators declare reads, writes, and
+additive/transformative behavior. `FVoxelOpStack::Add` snapshots that metadata, and
+`ValidateChannelOrder` checks the resulting channel DAG without entering the voxel loop.
+`VoxelForge.OpStack.ChannelDAG` builds all eight shipping stacks and validates them: **22 tests
+succeeded, 0 failed, 0 not run**.
+
 **⭐ Root polarity is a one-bit identity lever.** FloatingIslands is Maze's op classes rooted in a
 **void** source with a **fill** instead of a rock source with a carve — and you get islands instead of
 tunnels.
@@ -114,8 +125,17 @@ into one op with two internal phases. That removes the invalid state instead of
 ops that compose in any legal order beat eighteen with a footnote. (Fusing with identical order and
 math leaves the field bit-identical.)
 
+**Current implementation finding:** the fuse is not landed. Ten operators intervene between 4b and
+4h — `FCaveTerraceMod`, `FLayerLineMod`, `FRibbingMod`, `FCaveOverhangMod`, `FCaveCliffMod`,
+`FScallopMod`, `FCaveArchMod`, `FRoomColumnMod`, `FDomeMod`, and `FPinchMod` — and every one reads
+and writes `Density`. Moving the floor phase next to roughness would therefore reorder the live
+density updates; the required bit-identical gate cannot be assumed. The current 20-op stack remains
+in its original order until a fusion that preserves that order and math is available.
+
 **One declaration covers the rest:** an op is **additive** (small displacement, commutes freely — shuffle
 at will) or **transformative** (clamps, multiplies, gates — position matters, needs explicit placement).
+“Commutes” here is algebraic: IEEE-754 float accumulation is not generally bitwise commutative, so a
+shuffle still needs the equivalence gate.
 
 **The structure roll:**
 `root polarity -> shape source -> conversion (follows from polarity) -> draw k modifiers legal in the
@@ -485,8 +505,10 @@ they became "findings" (§13).
 (fall exposure, openness, traversal mix, tortuosity, corpus-centroid distance).
 
 ### Tier 3 — op-system prerequisites for free composition
-- **Channel read/write declarations** on every op, so ordering derives from a DAG (§3.2).
-- **Fuse corrective ops** (`FFloorBiasMod` into `FCaveRoughnessMod`).
+- ✅ **Channel read/write declarations** on every op, plus the stack DAG validator (§3.2; built
+  2026-09-04).
+- ⏸ **Fuse corrective ops** (`FFloorBiasMod` into `FCaveRoughnessMod`) — blocked by the ten
+  intervening `Density` readers/writers under the bit-identity requirement; see §3.2.
 - ⚠️ **`EffectOverBox` correct in ISOLATION** (§6.1) — the gate on everything after.
 - **The XY edge seal** (§6.7).
 
@@ -560,6 +582,12 @@ DiffLayerContention · FloatingIslandEquivalence · LargeSeedSurvives · **Layou
 LiveEditInvalidation · MazeEquivalence · SlabEquivalence · **StrateParamBlendCoverage** ·
 SurfaceHeightEquivalence · TunnelNetworkSpineEquivalence · VerticalShaftEquivalence.
 
+**2026-09-04 — Tier 3b validation run.** The specified UE 5.7 `VoxelMEditor Win64 Development`
+build succeeded (14 actions). The full headless suite reported **22 succeeded, 0 failed, 0 not run,
+0 succeeded-with-warnings**, including `VoxelForge.OpStack.ChannelDAG`; the existing equivalence,
+determinism, box-verdict, and connectivity checks stayed green. The 4b→4h fuse was deliberately
+not applied after the intervening-channel audit above.
+
 **⭐ This closes the pending-verification thread opened by `OPSTACK-HANDOFF.md`.** That file flagged
 commits `4d33321` (Sol's boundary fold) and `91585ea` as *built but NOT re-verified*, with two
 acceptance conditions. Both now met: