fix #426: extract solid-colour (NO_POS_UVS) faces; skip untextured subsets only on building shells and cells like retail

The Holtburg windmill axle (GfxObj 0x010010CE, 8 polygons, all
Stippling.NoPos + SurfaceType.Base1Solid) extracted to a 0-vertex mesh.
NoPos ("NO_POS_UVS", acclient.h:7380-7388) means "this side has no
texture coordinates" — true of every solid-colour polygon, since
nothing samples them — not "there is no positive face". Extraction read
it as the latter and dropped the polygon entirely, client-wide, for
every untextured polygon on every object.

Retail's D3DPolyRender::DrawMesh (@0x0059d4a0, named-retail decomp
~line 426048) draws an untextured subset on an ordinary object exactly
like a textured one; the only retail cases that skip an untextured
subset are a building shell (RenderDeviceD3D::DrawBuilding @0x0059f2a0
sets ObjBuildingOrBuildingPart=1) or an EnvCell interior
(RenderDeviceD3D::DrawEnvCell @0x0059f170, arg4=1). The #119
investigation's "retail's skipNoTexture never draws them either"
conclusion was itself wrong as a general rule.

- MeshExtractor.PrepareGfxObjMeshData / GfxObjMesh.Build: emit the
  positive side whenever PosSurface is a valid index, regardless of
  NoPos; the existing UV-index-0 fallback already produces zero
  texcoords for a NoPos polygon with no UVs on the wire.
- RetailUntexturedSurfacePolicy.IsUntextured(SurfaceType): the one
  place that answers "is this surface textured"
  ((type & (Base1Image|Base1ClipMap)) == 0), replacing the old
  `isSolid = NoPos || Base1Solid` (which also mis-classified a NEG-side
  batch by the POS-side's NoPos flag).
- RetailUntexturedSubsetPolicy.Draws(isBuildingShell, isUntextured):
  the shared draw-time gate wired into WbDrawDispatcher.ClassifyBatches,
  .PackedOracle.ClassifyPackedBatches, and
  .DirectionalShadows.AddDirectionalShadowBatches — one predicate so the
  three walks cannot drift (Campaign VM VM6 lesson).
- CellMesh.cs / MeshExtractor.PrepareCellStructMeshData deliberately
  KEEP their NoPos-gated skip for cell-wall geometry — retail's
  DrawEnvCell really does skip untextured subsets there; register row
  AP-234 documents the NoPos-vs-Surface.Type approximation.
- PakFormat.CurrentBakeToolVersion 4->5 (LauncherInstallRecordStore in
  lockstep): a pak baked by an older tool is missing every untextured
  face. No bake was run as part of this commit.

Also fixed: WorldBuilder's own upstream ObjectMeshManager.cs has the
identical NoPos bug (ObjectMeshManager.cs:959,984) — our port had
faithfully carried it over, and our own conformance test
(Build_NoPosFlag_OnlyEmitsNegSide) asserted the bug as correct WB
conformance. Renamed/reworded to Build_NoPosFlag_EmitsBothPosAndNegSide
with a citation for why retail decomp overrides WB here.

Issue119UpNullGfxObjDumpTests re-run against the installed DAT:
#119's own two objects (0x010002B4 9/9 polys, 0x010008A8 1/1 poly) now
gate DRAWS on every polygon instead of extracting to nothing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/AcDream.Core/Meshing/CellMesh.cs

@@ -41,7 +41,19 @@ public static class CellMesh
             if (poly.VertexIds.Count < 3)
                 continue;  // degenerate polygon
 
-            // Skip if NoPos stippling is set (polygon has no positive surface geometry).
+            // Retail's RenderDeviceD3D::DrawEnvCell (@0x0059f170) calls
+            // D3DPolyRender::DrawMesh with arg4=1, which skips every
+            // UNTEXTURED subset inside an EnvCell interior — unlike ordinary
+            // objects, which draw them (see RetailUntexturedSurfacePolicy /
+            // RetailUntexturedSubsetPolicy, #426). We approximate
+            // "untextured" here with the polygon's own NoPos stippling flag
+            // rather than resolving the Surface's own Type
+            // (Base1Image/Base1ClipMap) before this per-polygon decision —
+            // see docs/architecture/retail-divergence-register.md AP-234. Do
+            // NOT remove this gate the way #426 removed the matching gate in
+            // GfxObjMesh.Build/MeshExtractor.PrepareGfxObjMeshData — retail
+            // genuinely skips untextured cell geometry, unlike ordinary
+            // objects.
             if (poly.Stippling.HasFlag(DatReaderWriter.Enums.StipplingType.NoPos))
                 continue;