docs(render): ARCHITECTURE RESET — indoor render is a 3-gate patchwork; handoff + unified-PView target

A week on the indoor render (Phase U.4 → U.4c → 2026-05-31) fixed the flap but
produced NO shippable progress: walls/ceiling don't seal, outdoor terrain is
visible from inside (#78), the enclosure reads grey/transparent. Root cause is
ARCHITECTURAL, not a bug.

Evidence this session (direct, via the new [shell] probe + screenshots) RULED OUT
every subsystem except the gating architecture: the interior cell shells render
fine (geometry/texture/opaque/depth all correct, zh=0 tr=0); the visibility
traversal computes correct sets + non-empty portal clips; cull mode is fine; the
camera/eye thread was a detour. The residual is that OUTDOOR geometry is not gated
to portal openings when indoors, and acdream enforces visibility THREE inconsistent
ways (TerrainClipMode / per-cell shell clip / entity ParentCellId filter with an
outdoor-stab bypass) instead of retail's ONE PView gate.

This commit is the reset handoff + documentation, not a code fix:
- docs/research/2026-05-31-render-architecture-reset-handoff.md — canonical: honest
  state, evidence ledger (ruled-out / do-not-repeat), the mapped 3-gate patchwork,
  the retail PView target (one traversal → one gate for ALL geometry), the reset
  mission, and a copy-paste pickup prompt.
- docs/architecture/acdream-architecture.md — new "Render Pipeline" SSOT section
  (current divergence + unified-PView target + the one rule: compute visibility
  once, enforce it once). (Doc has pre-existing corruption below this section —
  flagged for separate cleanup.)
- Apparatus: ACDREAM_PROBE_SHELL → [shell] (EnvCellRenderer per-cell prepared/drawn
  geometry + flags) added to RenderingDiagnostics + EnvCellRenderer. Throwaway.
- docs/superpowers/specs/2026-05-31-camera-collision-indoor-engagement-design.md —
  spec for e099b4c (camera collision; now parked as orthogonal to the seam).

Next session: STOP point-fixing; do the architecture reset to a single PView gate.

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

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -114,6 +114,21 @@ public static class RenderingDiagnostics
     public static bool ProbeFlapEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_FLAP") == "1";
 
+    /// <summary>
+    /// Issue #78 (2026-05-31) cell-shell render probe. When true,
+    /// <c>EnvCellRenderer.Render</c> emits one <c>[shell]</c> line per opaque-pass
+    /// call: per visible (filtered) cell — is it present in the prepared snapshot,
+    /// how many gfxObjs + instances, and per-gfxObj batch count / index count /
+    /// translucent / zero-bindless-handle (missing texture) — plus the pass totals.
+    /// This directly answers WHY interior walls/ceilings don't appear: no geometry
+    /// prepared for the cell (cell absent / 0 instances), drawn-but-invisible
+    /// (zeroHandle / translucent against the clear color), or prepared+drawn (so the
+    /// fault is elsewhere — depth/occlusion). Throwaway apparatus — strip once the
+    /// indoor-enclosure render is fixed. Initial state from <c>ACDREAM_PROBE_SHELL=1</c>.
+    /// </summary>
+    public static bool ProbeShellEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_SHELL") == "1";
+
     // Cell-change gate for EmitVis. The probe fires once per distinct root cell
     // so launch.log stays readable under motion (the per-frame call is a no-op
     // when the root is unchanged). Sentinel 0 = "no root yet" — the first real