diag(render/physics): flap root-caused to physics rest µm-jitter; refute prior diagnoses

Apparatus + handoff for the indoor flap. Confirmed (primary evidence): the flap is the
portal-flood clip being µm-sensitive at the threshold, driven by a ~1-8µm jitter in the
player RenderPosition (physics resting position not bit-stable; Lerp surfaces it). REFUTES
the 2026-06-07 see-through/EnvCell/outdoor-node diagnosis (ModelId GfxObj 0x01000A2B IS the
solid exterior) AND an enqueue-once attempt (retail propagates late slices via AddToCell;
the existing PropagatesNewSlicesToExit test caught it; reverted). Adds: Build determinism
test, A8CellAudit gfxobj dump, [pv-input] 6dp probe + [render-sig] outRoot/bshell fields.
No functional fix shipped. Next: higher-precision physics rest trace -> port retail
kill_velocity/contact rest-stability. Canonical: docs/research/2026-06-08-flap-rootcause-physics-rest-handoff.md

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

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -129,6 +129,20 @@ public static class RenderingDiagnostics
     public static bool ProbeShellEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_SHELL") == "1";
 
+    /// <summary>
+    /// Flap root-cause apparatus (2026-06-07). When true, the indoor render path emits ONE
+    /// <c>[pv-input]</c> line per frame with the EXACT PortalVisibilityBuilder.Build inputs at HIGH
+    /// precision (camera eye + player position to 6 dp, plus orientation-sensitive view-projection
+    /// elements) alongside the resulting flood cell count. The live flap shows the flood set flipping
+    /// 2↔6 at an eye/player that is identical to cm; this probe answers whether the Build INPUTS differ
+    /// below cm precision (sub-cm view jitter → robustness fix) or are byte-identical while the output
+    /// still flips (nondeterminism → surgical bug). Runs WITHOUT the heavy <c>[flap]</c>/<c>[render-sig]</c>
+    /// spam so the log stays diffable. Throwaway apparatus — strip once the jitter source is pinned.
+    /// Initial state from <c>ACDREAM_PROBE_PVINPUT=1</c>.
+    /// </summary>
+    public static bool ProbePvInputEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_PVINPUT") == "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