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>

src/AcDream.Core/Lighting/LightManager.cs

@@ -37,7 +37,6 @@ namespace AcDream.Core.Lighting;
 public sealed class LightManager
 {
     public const int MaxActiveLights = 8;   // D3D parity
-    private const float RangeSlack = 1.1f;  // 10% hysteresis around hard cutoff
 
     private readonly List<LightSource> _all = new();
     private readonly LightSource?[] _active = new LightSource?[MaxActiveLights];
@@ -109,8 +108,16 @@ public sealed class LightManager
             Vector3 delta = light.WorldPosition - viewerWorldPos;
             light.DistSq = delta.LengthSquared();
 
-            float rangeSq = light.Range * light.Range * RangeSlack * RangeSlack;
-            if (light.DistSq > rangeSq) continue;
+            // Retail D3D-style fixed-pipeline lighting picks the 8 NEAREST point
+            // lights and applies each light's hard range-cutoff PER SURFACE in the
+            // shader (mesh_modern.frag: `if (d < range && range > 1e-3)`). The
+            // previous viewer-range candidacy filter (skip when DistSq > Range²·slack²)
+            // was wrong — it dropped a torch whenever the VIEWER stood outside that
+            // torch's range, so a dungeon room with 2227 registered torches lit only
+            // the ~1 the player was standing inside (activeLights≈1, the rest of the
+            // room at flat 0.2 ambient — the "dungeon lighting off" report). Take the
+            // nearest 8 regardless of viewer range; the shader's per-fragment
+            // `d < range` does the actual hard cutoff.
             candidates.Add(light);
         }