diag(render): composite-warmup stall probe for the session-3 tunnel hang

The 2026-07-29 Coldeve session 3 stuck the player in the portal tunnel
forever: generation 2 (Town Network, 0x00070156) published render but
composites/collision never became ready, and the reveal latch correctly
held the tunnel. The composite warmup queue in WbDrawDispatcher has
exactly two permanent-stall shapes - a GfxObj id that never resolves
(silent load failure, e.g. custom-server content absent from the baked
pak) or an upload budget that never reopens - and they are
indistinguishable from the reveal log alone.

ACDREAM_PROBE_REVEAL=1 (NetDiagnostics.ProbeReveal) now emits one
[composite-warmup] STALL line per second while warmup blocks a reveal:
pending count, queue depth, scan state, upload-budget gate, and the
first four pending GfxObj ids. Zero cost when off.

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

src/AcDream.Core.Net/NetDiagnostics.cs

@@ -33,4 +33,16 @@ public static class NetDiagnostics
     /// </summary>
     public static bool ProbeNet { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_NET") == "1";
+
+    /// <summary>
+    /// <c>ACDREAM_PROBE_REVEAL=1</c> — #260 reveal-stall probe: while a
+    /// reveal destination's composite warmup is incomplete, emit one
+    /// <c>[composite-warmup]</c> line per second naming the pending queue
+    /// depth, scan state, upload-budget gate, and the first few unresolved
+    /// GfxObj ids. The two permanent-stall shapes (a mesh id that never
+    /// resolves vs. an upload budget that never reopens) are otherwise
+    /// indistinguishable and invisible.
+    /// </summary>
+    public static bool ProbeReveal { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_REVEAL") == "1";
 }