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.App/Rendering/Shaders/mesh_modern.frag

@@ -16,6 +16,7 @@ in flat uint  vTextureLayer;
 //                         alpha < 0.05 (skip empty fragments — large
 //                         transparent overdraw cost otherwise)
 uniform int uRenderPass;
+uniform int uLightDebug;   // #176 stripe hunt (see mesh_modern.vert) — mode 3 handled here
 
 // SceneLighting UBO — IDENTICAL layout to mesh_instanced.frag binding=1.
 struct Light {
@@ -85,6 +86,14 @@ void main() {
     // Per-vertex Gouraud lighting from the vertex shader (ambient + capped lights).
     vec3 lit = vLit;
 
+    // #176 stripe-hunt mode 3: show the raw per-vertex light field (texture
+    // ignored). Stripes visible HERE = a vertex-lighting artifact; absent =
+    // the pattern comes from texture/per-pixel machinery. Throwaway diagnostic.
+    if (uLightDebug == 3) {
+        FragColor = vec4(min(lit, vec3(1.0)), 1.0);
+        return;
+    }
+
     // Lightning flash — additive scene bump (matches mesh_instanced.frag).
     lit += uFogParams.z * vec3(0.6, 0.6, 0.75);