fix(render): FW4 slice 2 - exit seals stamp the walk''s own flood

The cathedral falls shine-through (owner report at the #456 terrace
seam, toggling with the camera''s side of the seam): the probe''s new
phase tags pinned the falls emitters to the cell-owners route
submitting POST-depth-clear on interior roots - retail''s own timing,
safe only because seals own the aperture depth by the end-of-frame
alpha drain. Our seal SET still came from the old apparatus''s flood,
which misses exit portals at the #456 seam band (its never-drawn panel
family), so the drain z-passed across the unsealed opening.

DrawWalkExitPortalMasks now stamps the walk''s own ordered interior
flood (WalkFrameDriver.InteriorFloodCells, captured at
OnInteriorFloodDrawTurn - retail PView::DrawCells pc:432785-432786),
keeping the old assembly''s per-cell slice clips where present and
sealing unclipped where the old apparatus had no entry (benign: the
exact dat aperture polygon, z-tested). The legacy path keeps the old
seal draw. Also: ACDREAM_PROBE_WALK_ROOT gains [walk-part]/[walk-alpha]
phase-tagged submission/drain lines (the apparatus that pinned this).

Hermetic 6,762/0, Walk lane 213/1, InstalledDat walk conformance 40/1.

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

src/AcDream.Core/Rendering/RenderingDiagnostics.cs

@@ -182,6 +182,13 @@ public static class RenderingDiagnostics
     public static bool ProbeWalkRootEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_WALK_ROOT") == "1";
 
+    /// <summary>Companion scratch for <see cref="ProbeWalkRootEnabled"/>
+    /// (throwaway, same investigation): the frame phase relative to the
+    /// interior depth clear ("pre"/"post"), stamped by the pass executor so
+    /// particle/alpha probe lines can tag WHEN a submission or drain ran.
+    /// Written only while the probe is on.</summary>
+    public static string WalkRootPhase = "";
+
     /// <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,