diag #176/#180: isolation toggles pin the residual stripes on the camera sweep, not the render

The #176 gate-2 failure ("stripes/triangles flickering when the camera
is pushed into walls; nothing when zoomed out") is NOT a render defect.
Isolation apparatus added this commit:

- ACDREAM_LIGHT_DEBUG shader modes (mesh_modern.vert/frag + uLightDebug
  upload in EnvCellRenderer/WbDrawDispatcher): 1 = ambient-only,
  2 = dynamics killed, 3 = raw vLit field (texture ignored). The
  pattern SURVIVES mode 3 -> not texture; lives above the light data.
- ACDREAM_CLIP_DEBUG=1 (RenderingDiagnostics.ClipDebugNoShellTrim +
  the EnvCellRenderer slot-fill gate): shell pass draws cells WHOLE
  (retail's shape). Pattern survives -> the per-cell clip trim is
  exonerated.

With every render suspect dead, an autonomous visual loop (synthetic
back-into-wall input + GDI window captures + the [flap-sweep] probe)
pinned the mechanism numerically: at a compressed moving boom the
camera-collision sweep is BISTABLE - consecutive sweeps with ~1.4 mm
input drift flip the first-contact solution 0.27 m along the boom
(pulledIn 0.27<->0.53, every ~5-10 frames, all 368k sweeps ok=True),
and at ~1700 fps unsynced every monitor refresh tear-interleaves the
two views = the stripe/hatch patterns. Filed as #180 with the retail
anchor: viewer_sought_position is STATEFUL (SmartBox 0x00452d75 feeds
the CURRENT swept viewer into CameraManager::UpdateCamera 0x00456660
and assigns the return to the sought, 0x00452d84) - the target
converges to the collided position instead of re-rolling the full
knife-edge ray per frame like our RetailChaseCamera does. SweepEye
itself ports update_viewer 0x00453ce0 faithfully and is exonerated.

Also recorded in #176: the site-A weenie light-registration leak (a
portal's I100 light stacked x2->x4 over one session as re-CreateObject
re-registered it under fresh entity ids).

The #176 lighting fix (d8984e87) remains live-verified; #176 re-gates
after #180 lands. ISSUES: #180 filed, #176 updated. Suites: Core
2599+2skip; toggles inert by default.

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

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -336,6 +336,36 @@ public static class RenderingDiagnostics
     public static IReadOnlySet<uint> SeamDrawTargetCells { get; } =
         ParseSeamDrawTargets(Environment.GetEnvironmentVariable("ACDREAM_PROBE_SEAMDRAW"));
 
+    /// <summary>
+    /// #176 stripe-hunt shader isolation mode (<c>ACDREAM_LIGHT_DEBUG</c>) —
+    /// throwaway diagnostic, uploaded as <c>uLightDebug</c> by EnvCellRenderer +
+    /// WbDrawDispatcher each pass. 0 = off; 1 = ambient-only vertex lighting
+    /// (all point/sun contributions killed); 2 = DYNAMIC point lights killed
+    /// (the intensity-100 portal purples + the viewer fill off; statics stay);
+    /// 3 = raw vLit visualization in the fragment shader (texture ignored).
+    /// Discriminates lighting-driven stripes (gone at 1/2, visible in the field
+    /// at 3) from texture/per-pixel machinery (survive 1). Settable for a
+    /// future DebugPanel mirror.
+    /// </summary>
+    public static int LightDebugMode { get; set; } =
+        int.TryParse(Environment.GetEnvironmentVariable("ACDREAM_LIGHT_DEBUG"), out var ldm) ? ldm : 0;
+
+    /// <summary>
+    /// #176 stripe-hunt isolation (<c>ACDREAM_CLIP_DEBUG=1</c>) — throwaway
+    /// diagnostic. When true, the EnvCell SHELL pass maps every instance to clip
+    /// slot 0 (the reserved no-clip region) instead of its cell's portal-slice
+    /// region — i.e. shells draw WHOLE, retail's shape (retail never clips cell
+    /// geometry; aperture exactness comes from the seal/punch depth writes +
+    /// far→near order, PView::DrawCells 0x005a4840). Discriminator for the
+    /// camera-against-wall stripe/hatch pattern: a knife-edge slice region (eye
+    /// on a portal plane / inside a wall) yields a clip plane near-parallel to
+    /// large surfaces → interpolated gl_ClipDistance ≈ 0 across them → per-pixel
+    /// sign dither = the hatch. Stripes gone with this on = the shell trim is
+    /// the mechanism. Entity/dispatcher clip routing is NOT affected.
+    /// </summary>
+    public static bool ClipDebugNoShellTrim { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_CLIP_DEBUG") == "1";
+
     /// <summary>Parse ACDREAM_PROBE_SEAMDRAW: "1"/"true"/empty → the default #176
     /// Facility Hub set; otherwise a comma-separated hex cell-id list (same forgiving
     /// grammar as <see cref="ParseDumpEntityIds"/>). Internal for unit tests.</summary>