chore(repo): archive historical docs under Docs/archive, drop tracked pycache

Move closed audits, task briefs, design rounds, perf reports, the OPSTACK
plan/progress logs and WORK-NEXT.md into Docs/archive/ (git mv, history kept).
Whitelist Docs/ in the strict .gitignore, ignore Tools/**/__pycache__/, and
remove the tracked .pyc. Repoint references in the kept docs and in non-test
source comments to the new paths.

Source/VoxelForge/Public/VoxelDensityOp.h

@@ -1,6 +1,6 @@
 // VoxelDensityOp.h
 // LE CONTRAT de la pile d'opérateurs de densité / THE density operator stack CONTRACT.
-// Phase 1 de OPSTACK-PLAN.md. The contract is implemented by the opt-in stack path in
+// Phase 1 de Docs/archive/OPSTACK-PLAN.md. The contract is implemented by the opt-in stack path in
 // UVoxelGenerator::GetDensityAt; the legacy archetype switch remains for non-opt-in strates.
 //
 // ─────────────────────────────────────────────────────────────────────────────────────────

Source/VoxelForge/Public/VoxelStrateDefinition.h

@@ -128,7 +128,7 @@ public:
      *
      * ⚠️ Do NOT flip this on a strate mid-session and expect the old and new geometry to agree to
      * the bit — they differ by ~1-2 ULP with ZERO isosurface crossings, so the shape is identical
-     * but the floats are not (`AUDIT-2026-07.md §C10`). Regenerate the world after changing it
+     * but the floats are not (`Docs/archive/AUDIT-2026-07.md §C10`). Regenerate the world after changing it
      * rather than letting old and new tiles sit side by side.
      *
      * Pile d'opérateurs (expérimental) : génère cette strate via la pile composable au lieu du

Source/VoxelForge/VoxelForge.Build.cs

@@ -26,7 +26,7 @@ public class VoxelForge : ModuleRules
 		//
 		// Precise resolves to "/fp:precise" on MSVC and "-ffp-contract=off" on Clang — both
 		// IEEE-754 compliant with no FMA contraction, so the two toolchains agree BY CONSTRUCTION
-		// rather than by luck. That is the fix for AUDIT-2026-07.md C9.
+		// rather than by luck. That is the fix for Docs/archive/AUDIT-2026-07.md C9.
 		//
 		// COST: /fp:precise forbids the reassociation and contraction /fp:fast allowed, on a
 		// noise-heavy hot path. Expect a measurable perf regression and check it against