feat(render): Phase A8 Wave 5 — probe trail ([envcells]/[stencil]/[draworder]/[buildings])

Probe emitters wired (replaces the Task 8 stubs). All gated on
ACDREAM_PROBE_VIS=1 (everything) or ACDREAM_PROBE_ENVCELL=1
([envcells] only):

- [envcells] frame=N cells=N tris=N ourBldgs=N otherBldgs=N filterCnt=N
  Fires once per Render call inside RenderInsideOutAcdream Step 3.
  Reads CellsRendered + TrianglesDrawn from EnvCellRenderer.Stats.

- [stencil] op={mark|punch} bld=0xHHHHHHHH verts=N
  Fires after every IndoorCellStencilPipeline.RenderBuildingStencilMask
  call (Steps 1, 2, 5a, 5b, 5d) — surfaces LastStencil* probe fields
  added in Wave 1's Task 7 extension.

- [draworder] frame=N step=Xy stencil={on|off} depthFn=0xHHH depthMask={true|false}
  Fires at each step boundary (entry to Step 1/2/3/4/5{a,b,c,d}).
  Reads live GL state via glGetInteger so divergence between assumed
  vs actual state is immediately visible.

- [buildings] camCell=0xHHHHHHHH camBldgs=[0x1,0x2,...] otherBldgs=N totalKnown=N
  Fires once per indoor frame at the top of RenderInsideOutAcdream.
  totalKnown sums BuildingRegistry.Count across all loaded landblocks.

Per-frame counter _phaseA8DrawOrderFrame incremented once per render
tick after the existing [vis] probe block (line 7104).

New env-var flag ACDREAM_PROBE_ENVCELL in RenderingDiagnostics +
ProbeEnvCellEnabled property (true OR ProbeVisibilityEnabled).

Mandatory acceptance criteria (process rule "no visual-gate launch
without probe data first") to check FROM the log BEFORE asking the
user for visual verification:
  - [buildings] camBldgs=[0x...] non-empty when inside a cottage
  - [envcells] cells>=1 tris>=1 filterCnt>=1 for at least one indoor frame
  - [stencil] op=mark verts>0 fires per camera-building
  - [draworder] shows the full Step 1 → 2 → 3 → 4 → 5{a,b,c,d} cycle

Build green. 82/82 App.Tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -86,6 +86,23 @@ public static class RenderingDiagnostics
     public static bool ProbeVisibilityEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_VIS") == "1";
 
+    private static bool _probeEnvCellEnabled =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_ENVCELL") == "1";
+
+    /// <summary>
+    /// Phase A8 Task 9 (2026-05-28): when true, RenderInsideOutAcdream's
+    /// <c>[envcells]</c> probe emits one line per indoor frame —
+    /// CellsRendered / TrianglesDrawn from <see cref="AcDream.App.Rendering.Wb"/>'s
+    /// <c>EnvCellRenderer.Stats</c> + ourBldgs/otherBldgs/filterCnt.
+    /// Also enabled implicitly when <see cref="ProbeVisibilityEnabled"/> is true.
+    /// Initial state from <c>ACDREAM_PROBE_ENVCELL=1</c>.
+    /// </summary>
+    public static bool ProbeEnvCellEnabled
+    {
+        get => _probeEnvCellEnabled || ProbeVisibilityEnabled;
+        set => _probeEnvCellEnabled = value;
+    }
+
     /// <summary>
     /// Master toggle. Reading reflects the AND of all five flags
     /// (true only when every probe is on). Writing cascades — setting