feat(core+app): Phase 7.1 — render EnvCell room geometry (walls/floors/ceilings)

Interior walls, floors, and ceilings were invisible because the Phase 2d
walker only consumed StaticObjects and skipped each cell's CellStruct
(VertexArray + Polygons + EnvCell.Surfaces). This commit ports the same
fan-triangulated per-surface bucket pattern from GfxObjMesh into a new
CellMesh module, then wires it into the interior walker so each EnvCell
now contributes both its static props and its room mesh. The cell's world
transform (rotation * translation(cellOrigin + lbOffset)) is baked into
MeshRef.PartTransform with WorldEntity at identity, matching how
StaticMeshRenderer composes model = PartTransform * entityRoot.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Meshing/CellMesh.cs

@@ -0,0 +1,107 @@
+using System.Numerics;
+using AcDream.Core.Terrain;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Types;
+
+namespace AcDream.Core.Meshing;
+
+/// <summary>
+/// Builds renderable sub-meshes from an EnvCell's room geometry (walls,
+/// floors, ceilings). The geometry lives in the linked Environment dat:
+/// EnvCell.EnvironmentId → Environment → Cells[CellStructure] → CellStruct.
+/// This mirrors GfxObjMesh.Build but reads surfaces from EnvCell.Surfaces
+/// (not from the CellStruct itself) and uses the same fan-triangulation
+/// and per-surface deduplication pattern.
+/// </summary>
+public static class CellMesh
+{
+    /// <summary>
+    /// Walk a CellStruct's polygons and produce one <see cref="GfxObjSubMesh"/>
+    /// per referenced Surface. Surfaces are resolved from <paramref name="envCell"/>.Surfaces
+    /// (OR'd with 0x08000000 to form the full dat id). Polygons are triangulated as fans.
+    /// </summary>
+    public static IReadOnlyList<GfxObjSubMesh> Build(EnvCell envCell, CellStruct cellStruct)
+    {
+        // Group output vertices and indices per surface dat id.
+        var perSurface = new Dictionary<uint, (List<Vertex> Vertices, List<uint> Indices, Dictionary<(int pos, int uv), uint> Dedupe)>();
+
+        foreach (var kvp in cellStruct.Polygons)
+        {
+            var poly = kvp.Value;
+
+            if (poly.VertexIds.Count < 3)
+                continue;  // degenerate polygon
+
+            // Skip if NoPos stippling is set (polygon has no positive surface geometry).
+            if (poly.Stippling.HasFlag(DatReaderWriter.Enums.StipplingType.NoPos))
+                continue;
+
+            int surfaceIdx = poly.PosSurface;
+            if (surfaceIdx < 0 || surfaceIdx >= envCell.Surfaces.Count)
+                continue;  // out-of-range surface index
+
+            // Surfaces on EnvCell are unqualified ids; OR with 0x08000000 for the full dat id.
+            uint surfaceId = (uint)envCell.Surfaces[surfaceIdx] | 0x08000000u;
+
+            if (!perSurface.TryGetValue(surfaceId, out var bucket))
+            {
+                bucket = (new List<Vertex>(), new List<uint>(), new Dictionary<(int, int), uint>());
+                perSurface[surfaceId] = bucket;
+            }
+
+            // Collect output vertex indices for this polygon.
+            var polyOut = new List<uint>(poly.VertexIds.Count);
+            bool skipPoly = false;
+
+            for (int i = 0; i < poly.VertexIds.Count; i++)
+            {
+                int posIdx = poly.VertexIds[i];
+                int uvIdx = i < poly.PosUVIndices.Count ? poly.PosUVIndices[i] : 0;
+
+                if (!cellStruct.VertexArray.Vertices.TryGetValue((ushort)posIdx, out var sw))
+                {
+                    skipPoly = true;
+                    break;
+                }
+
+                var texcoord = uvIdx >= 0 && uvIdx < sw.UVs.Count
+                    ? new Vector2(sw.UVs[uvIdx].U, sw.UVs[uvIdx].V)
+                    : Vector2.Zero;
+
+                // Use normal from vertex data; fall back to up-vector if missing.
+                var normal = sw.Normal != Vector3.Zero ? sw.Normal : Vector3.UnitZ;
+
+                var key = (posIdx, uvIdx);
+                if (!bucket.Dedupe.TryGetValue(key, out var outIdx))
+                {
+                    outIdx = (uint)bucket.Vertices.Count;
+                    bucket.Vertices.Add(new Vertex(sw.Origin, normal, texcoord, TerrainLayer: 0));
+                    bucket.Dedupe[key] = outIdx;
+                }
+                polyOut.Add(outIdx);
+            }
+
+            if (skipPoly || polyOut.Count < 3)
+                continue;
+
+            // Fan triangulation: (v0, v1, v2), (v0, v2, v3), ...
+            for (int i = 1; i < polyOut.Count - 1; i++)
+            {
+                bucket.Indices.Add(polyOut[0]);
+                bucket.Indices.Add(polyOut[i]);
+                bucket.Indices.Add(polyOut[i + 1]);
+            }
+        }
+
+        // Emit one sub-mesh per surface.
+        var result = new List<GfxObjSubMesh>(perSurface.Count);
+        foreach (var kvp in perSurface)
+        {
+            result.Add(new GfxObjSubMesh(
+                SurfaceId: kvp.Key,
+                Vertices: kvp.Value.Vertices.ToArray(),
+                Indices: kvp.Value.Indices.ToArray()));
+        }
+        return result;
+    }
+}