docs(work-next): promote item 4 to performance architecture

Written by the classifier-fix Codex round before it was closed. The classifier is a
second evaluator -- interval proofs, exact certificates and final-field validation --
beside the canonical lowered one, and the runtime can validate the same tile again
before meshing. Any classifier kept long-term must consume the lowered representation
and reusable density data.

That round's other uncommitted edit was NOT kept: it flipped
GVoxelForgeOuterClassifierMode 1 -> 0, turning the outer classifier off by default.
That was A/B setup for an unfinished experiment, not a measured decision, so the
committed default (1, classifier on) is restored.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01X9DLqyXrJxG4TojzDrSmVw

WORK-NEXT.md

@@ -60,7 +60,7 @@ floor waits.
 **Do**: abort no-longer-desired tiles; sort from the true pawn position; give the occupied tile and the
 next tile along the heading absolute priority.
 
-## 4. Collapse the duplicated field semantics  ·  MAINTENANCE  ·  medium-large
+## 4. Collapse the duplicated field semantics  ·  PERFORMANCE ARCHITECTURE  ·  medium-large  
 
 The effective field now lives in three places: the operator stack, the classifier's parallel interval
 logic, and a post-stack repair tail in `GetDensityAt` (origin floors, passage structure, passage
@@ -72,6 +72,12 @@ operation sits in the pipeline. It also makes interval proofs harder to trust.
 
 **Do**: make the stack the single semantic source of truth. Likely overlaps heavily with item 2.
 
+**Promoted 2026-09-12**: this is performance architecture, not maintenance. The classifier is a
+second evaluator (interval proofs, exact certificates, and final-field validation) beside the
+canonical lowered evaluator, and the runtime can validate the same tile again before meshing.
+Items 1 and 2 in the current round deliberately leave this lowering work untouched; any
+classifier retained later must consume the lowered representation and reusable density data.
+
 ## 5. Verdict cache removed after moving-session audit  ·  complete
 
 The clean moving-session audit found zero outer or nested hits: 2,109 outer probes and 19,904 nested