fix(#79/#93): mesh-empty hydration gate was dropping light-only fixtures

Root-caused via dat-truth inspection, not inference: the A7.L1 visible-
cell scoping fix (previous commit) had zero visual effect because the
Town Network fountain room (cell 0x00070144) registers ZERO lights of
its own — confirmed directly against the dat, not assumed. The room's
one dat-authored fixture, Setup 0x02000365 (a ceiling light 5m above the
fountain, warm color, intensity 100), has a single visual part that is a
#136-class runtime-hidden marker. Flattened mesh ref count: 0. GameWindow
's per-stab hydration loop treated meshRefs.Count==0 as "doesn't exist"
and dropped the whole entity before the Setup's Lights were ever read —
so a mesh-less "light attach point" fixture, a normal AC dat authoring
pattern, could never register. Retail's light registration (add_light,
CEnvCell::UnPack) is architecturally independent of a fixture's own mesh
visibility.

Fix: track the stab's Setup.Lights.Count alongside meshRefs during
hydration; keep the entity (with empty MeshRefs — nothing to draw, still
something to light) whenever either is nonzero. Extracted the decision
into EntityHydrationRules.ShouldKeepEntity (pure, unit-tested) since
GameWindow's hydration loop isn't independently testable. Confirmed no
downstream consumer assumes MeshRefs.Count >= 1 (WbDrawDispatcher already
guards on it before any indexing).

Core 2666+2skip / App 741+2skip / UI 425 / Net 385 green. Apparatus:
Issue93TownNetworkFountainRoomLightInspectionTests (dat-truth dump,
reusable for other rooms in this class).

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

tests/AcDream.Core.Tests/Meshing/EntityHydrationRulesTests.cs

@@ -0,0 +1,38 @@
+using AcDream.Core.Meshing;
+using Xunit;
+
+namespace AcDream.Core.Tests.Meshing;
+
+/// <summary>
+/// #79/#93 (2026-07-09) — the Town Network fountain room's only dat-authored
+/// light (Setup 0x02000365, a ceiling fixture) never registered: its sole
+/// visual part is a #136-class runtime-hidden marker, so it flattens to zero
+/// mesh refs, and GameWindow.cs's hydration loop dropped the ENTIRE entity —
+/// light included — whenever meshRefs was empty. Retail's light registration
+/// (CObjCell::add_light, CEnvCell::UnPack) is entirely independent of a
+/// fixture's own mesh visibility; the mesh-empty gate must not also swallow
+/// a Setup's Lights.
+/// </summary>
+public class EntityHydrationRulesTests
+{
+    [Fact]
+    public void ShouldKeepEntity_NoMeshNoLights_False()
+    {
+        Assert.False(EntityHydrationRules.ShouldKeepEntity(meshRefCount: 0, setupLightCount: 0));
+    }
+
+    [Fact]
+    public void ShouldKeepEntity_NoMeshWithLights_True()
+    {
+        // The exact fountain-room case: a mesh-less "light attach point" must
+        // still survive hydration so its Lights can be registered.
+        Assert.True(EntityHydrationRules.ShouldKeepEntity(meshRefCount: 0, setupLightCount: 1));
+    }
+
+    [Fact]
+    public void ShouldKeepEntity_HasMesh_TrueRegardlessOfLights()
+    {
+        Assert.True(EntityHydrationRules.ShouldKeepEntity(meshRefCount: 1, setupLightCount: 0));
+        Assert.True(EntityHydrationRules.ShouldKeepEntity(meshRefCount: 3, setupLightCount: 2));
+    }
+}