diag(render): §4 flap apparatus — eye-vs-terrain field + GL-state tripwire

Two probe additions from the §4 outdoor full-world flap investigation
(world turns to fog-tinted clear color at specific outdoor spots;
strobes then holds; camera rotation recovers; forward-walk triggers):

- [pv-input] gains terrZ=/eyeAbove= (terrain height under the camera
  eye) — used to REFUTE the buried-eye hypothesis (eyeAbove stayed
  +1.6..2.1 m through every held-flap block).
- [gl-state] (ACDREAM_PROBE_GLSTATE=1): snapshots the GL fixed-function
  state entering the world passes (depth, blend, cull, scissor + box,
  viewport, FBO, color/clip masks, glGetError), printing on change.
  Delivered the frame-exact flap-onset marker: the leftover scissor box
  flips from full-screen to a drifting 9x21 px doorway footprint at the
  exact frame the nearby building flood merges in (pv-input flood 1->5).

Both probes are gated and zero-cost when off. Strip with the rest of
the §4 apparatus when the flap ships.

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

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -143,6 +143,18 @@ public static class RenderingDiagnostics
     public static bool ProbePvInputEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_PVINPUT") == "1";
 
+    /// <summary>
+    /// §4 outdoor full-world flap apparatus (2026-06-09). When true, GameWindow snapshots the
+    /// GL fixed-function state entering the world passes each frame (depth test/mask/func, blend
+    /// + factors, cull, front-face, scissor + box, viewport, draw-FBO, color mask, glGetError)
+    /// and emits one <c>[gl-state]</c> line whenever the snapshot CHANGES. Pins or refutes the
+    /// "leaked GL state" family for the flap (every CPU-side input — matrix, flood, clip planes,
+    /// scissor box, membership, eye-vs-terrain — is already probe-exonerated). Throwaway
+    /// apparatus — strip once §4 ships. Initial state from <c>ACDREAM_PROBE_GLSTATE=1</c>.
+    /// </summary>
+    public static bool ProbeGlStateEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_GLSTATE") == "1";
+
     /// <summary>
     /// Bounded-propagation port apparatus (2026-06-08). When true, PortalVisibilityBuilder.Build emits
     /// one [portal-churn] summary line per call: per-cell pop count (re-pops = churn), total re-enqueues,