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

ARCHITECTURE.md

@@ -378,6 +378,11 @@ driven by `EditorBrush*` props.
   no loads AND no LOD mismatches outstanding.
 - **SDF cache** (`GetDensityWithParams`): search-BOX validity, not chunk-key — gradient ±1
   sampling must not thrash the (expensive) rebuild.
+- **Operator channel metadata** (`FVoxelOpStack`): `ChannelReads`, `ChannelWrites`, and
+  `IsAdditive` are called only when an operator is added to a stack and their values are cached in
+  the stack entry. `ValidateChannelOrder` is assembly/diagnostic-only; no declaration virtual may
+  enter `Eval`'s per-voxel loop. If `FVoxelOpSample` gains a field, add its channel bit and update
+  every operator declaration and the validator together.
 - **VerticalShafts field cache**: shaft rolls and connector decisions, including the structural
   tree and structural-spine connector endpoint, are rebuilt only when the thread-local centre cell
   or a geometry-affecting parameter changes. Each rebuild rolls a wider direct-indexed geometric