BR-2 task 1: phantom-site probe (ACDREAM_PROBE_PHANTOM)

The BR-1 pre-check left the #113 phantom residual with two surviving
suspects, both cell-side: (a) flood-admitted cells whose shell draws with
a pass-all slice (NoClipSlice fallback when the assembler handed no slot,
or an assembler slot-0 scissor-fallback slice), and (b) cell entity
buckets drawn unclipped + un-viewcone'd by design.

[phantom-shell]: per shell-pass cell, print-on-change - clip-enable
state, slot presence, every drawn slice's slot + plane count with
PASS-ALL flagged. [phantom-objs]: per object-list cell, print-on-change
- entity bucket size. Env-gated ACDREAM_PROBE_PHANTOM=1, zero cost off,
throwaway (strip when the phantom closes).

Repro protocol: launch with the probe on, stand at the hall bisect spot
(world ~216,-108 looking at the AAB3 meeting hall west face) where the
phantom is visible, read which mechanism fires for stair cells
0xAAB30100..0x106. Shells pass-all -> BR-2/BR-3 close it; statics ->
BR-5 closes it.

Build green; App suite green.

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

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -198,6 +198,24 @@ public static class RenderingDiagnostics
     public static bool ProbePortalChurnEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_PORTAL_CHURN") == "1";
 
+    /// <summary>
+    /// BR-2 phantom-site probe (2026-06-11; plan
+    /// <c>docs/plans/2026-06-11-building-render-port-plan.md</c> §BR-2 first
+    /// task). The BR-1 pre-check proved the #113 phantom residual cannot be
+    /// GfxObj portal fills (never extracted); the surviving suspects are
+    /// cell-side. When true, <c>RetailPViewRenderer</c> emits, print-on-change
+    /// per cell: <c>[phantom-shell]</c> — per shell-pass cell, the clip-enable
+    /// state and each drawn slice's slot + plane count, flagging the pass-all
+    /// cases (NoClipSlice fallback for slot-less cells; assembler slot-0
+    /// scissor fallback) — and <c>[phantom-objs]</c> — per object-list cell,
+    /// the entity-bucket size drawn unclipped/un-viewcone'd. Reproducing the
+    /// phantom with this on pins which mechanism draws it (shells → BR-2/BR-3;
+    /// statics → BR-5). Throwaway apparatus — strip when the phantom closes.
+    /// Initial state from <c>ACDREAM_PROBE_PHANTOM=1</c>.
+    /// </summary>
+    public static bool ProbePhantomEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_PHANTOM") == "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