fix(G.3 A7): dungeon lighting — select 8 NEAREST lights, not viewer-in-range (#133)

The active-light selection dropped any point light whose range didn't reach the
VIEWER (DistSq > Range^2*slack -> skip). Retail's D3D-style fixed pipeline picks
the 8 NEAREST lights and applies the hard range cutoff PER SURFACE in the shader
(mesh_modern.frag: if (d < range)). The viewer-range candidacy filter suppressed
a torch whenever the player stood outside its range, so a dungeon room with 2227
registered torches lit only the ~1 the player was standing in (activeLights ~= 1,
rest of the room at flat 0.2 ambient = the "lighting off" report). Drop the filter;
take the nearest 8 regardless of viewer range. Removed the now-unused RangeSlack
const; updated the two tests that codified the old filter. Core lighting suite green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/AcDream.Core.Tests/Lighting/LightManagerTests.cs

@@ -60,21 +60,29 @@ public sealed class LightManagerTests
     }
 
     [Fact]
-    public void Tick_DropsLightsOutsideRangeWithSlack()
+    public void Tick_SelectsByDistance_RegardlessOfViewerRange()
     {
+        // Retail D3D-style: candidacy is distance-only (the nearest 8). A torch
+        // lights its OWN surfaces — the shader applies the hard `d < range` cutoff
+        // PER FRAGMENT (mesh_modern.frag) — so a torch the VIEWER is standing
+        // outside the range of is still selected; it lights the wall it sits on.
+        // Replaces the old viewer-range candidacy filter that suppressed it, which
+        // left dungeon rooms (2227 registered torches) at activeLights≈1 / flat 0.2
+        // ambient — the "dungeon lighting off" report (#133 A7).
         var mgr = new LightManager();
-        mgr.Register(MakePoint(new Vector3(20, 0, 0), range: 5f));  // far outside its own range
+        mgr.Register(MakePoint(new Vector3(20, 0, 0), range: 5f));  // viewer outside the torch's range
 
         mgr.Tick(viewerWorldPos: Vector3.Zero);
 
-        Assert.Equal(0, mgr.ActiveCount);
+        Assert.Equal(1, mgr.ActiveCount);   // selected by distance; the shader culls per-surface
     }
 
     [Fact]
-    public void Tick_IncludesLightsNearRangeEdge_WithSlack()
+    public void Tick_IncludesNearbyLight()
     {
         var mgr = new LightManager();
-        // Light at distance 5.0, range 5.0: distSq=25, rangeSq*1.1^2 = 25*1.21 = 30.25 → included.
+        // A nearby point light is selected (distance-only candidacy; the shader
+        // applies the per-fragment range cutoff).
         mgr.Register(MakePoint(new Vector3(5, 0, 0), range: 5f));
 
         mgr.Tick(viewerWorldPos: Vector3.Zero);