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/GfxObjMesh.cs

@@ -28,8 +28,16 @@ public static class GfxObjMesh
     /// The rule for emitting a polygon side:
     /// </para>
     /// <list type="bullet">
-    ///   <item><b>Pos side:</b> emit whenever <c>!Stippling.NoPos</c> and
-    ///         <c>PosSurface</c> is a valid index.</item>
+    ///   <item><b>Pos side:</b> emit whenever <c>PosSurface</c> is a valid
+    ///         index, REGARDLESS of <c>Stippling.NoPos</c>. #426
+    ///         (2026-08-23): NoPos ("NO_POS_UVS", acclient.h:7386) means
+    ///         "this side has no texture coordinates" — every solid-colour
+    ///         polygon carries it, since it has no UVs to carry — not "there
+    ///         is no positive face". Retail's <c>D3DPolyRender::DrawMesh</c>
+    ///         draws untextured (solid) subsets on ordinary objects the same
+    ///         as textured ones (see <see cref="RetailUntexturedSurfacePolicy"/>);
+    ///         a NoPos polygon with no UVs to read falls back to UV index 0 /
+    ///         zero texcoords below.</item>
     ///   <item><b>Neg side:</b> emit when
     ///         <c>Stippling.Negative</c>, <c>Stippling.Both</c>, or
     ///         <c>(!Stippling.NoNeg &amp;&amp; SidesType == CullMode.Clockwise)</c>.
@@ -69,9 +77,10 @@ public static class GfxObjMesh
                 continue;  // degenerate — can't form a triangle
 
             // --- Positive side ---
-            bool hasPos = !poly.Stippling.HasFlag(StipplingType.NoPos);
-            if (hasPos)
-                EmitSide(poly, poly.PosSurface, isNeg: false);
+            // #426: always emit — NoPos only means "no positive UVs", not
+            // "no positive face" (see the class doc). EmitSide's own
+            // surfaceIdx-validity guard is the only real gate.
+            EmitSide(poly, poly.PosSurface, isNeg: false);
 
             // --- Negative side ---
             // Three ways AC flags a polygon as double-sided: