fix(render): §4 outdoor full-world flap — empty Transparent pass leaked DepthMask(false), no-oping the frame depth clear

ROOT CAUSE (pinned by the [clip-route] probe run + [gl-state] tripwire, one
capture): EnvCellRenderer.RenderModernMDIInternal established the Transparent
pass state (Enable(Blend) + DepthMask(false)) BEFORE the batch pass-filter, so
a cell whose batches are ALL opaque (a plain cottage interior) hit the
totalDraws==0 early-out and returned without ever reaching the end-of-pass
restore. The frame ended with dmask=0 + blend=1; the NEXT frame's
glClear(GL_DEPTH_BUFFER_BIT) silently no-oped (depth clears honor glDepthMask),
and every world fragment — terrain, entities, player, sky — failed GL_LESS
against its own previous-frame depth ghost. Screen = the fog-tinted clear
color. Onset locks to the building-flood merge because that is the first frame
the flooded building shell draws; holds while merged (the leak re-arms every
frame); camera rotation recovers because the cell drops from the flood and the
restore-skipping path stops running.

Capture evidence (flap-cliproute-capture.log): all three draw-input suspects
exonerated — landscape scissor full-screen all run, terrain-UBO/region-SSBO
planes full-screen on both sides of every merge, all 41,373 instances on the
correct repacked slot with cullEnt=0 — while [gl-state] showed frames entering
with dmask=0 blend=1 for exactly the merged stretches (145,238 consecutive
frames in the held window, flipping with each merge boundary at the end-of-run
strobe cycles).

Fix (all paths root-cause, no suppression):
- EnvCellRenderer: move the pass-state establish BELOW the totalDraws==0
  early-out so state is only set on a path that always reaches the restore;
  hoist the globalVao==0 check (the second leak-shaped early-out) above the
  state set.
- GameWindow frame clear: assert DepthMask(true) before glClear — the clear
  DEPENDS on the depth write mask, so it sets the state it depends on
  (feedback_render_self_contained_gl_state; this is the 4th instance of the
  class, in the same function as the 1st).

Very likely the same family as the "parts of the screen flash while running
past cottages" and cottage enter/exit artifacts (every brief merge = a
1-frame no-op depth clear). Visual gate pending.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -7093,6 +7093,15 @@ public sealed class GameWindow : IDisposable
             System.Math.Clamp(fogColor.Z, 0f, 1f),
             1f);
 
+        // §4 outdoor full-world flap (2026-06-10): the depth clear DEPENDS on the depth
+        // write mask — glClear(GL_DEPTH_BUFFER_BIT) is silently gated by glDepthMask.
+        // A pass that leaked DepthMask(false) at frame end (EnvCellRenderer's empty
+        // Transparent pass, fixed same-day) turned this clear into a no-op and the whole
+        // world failed GL_LESS against its own previous-frame depth ghost. Per
+        // feedback_render_self_contained_gl_state: the clear site asserts the state it
+        // depends on rather than inheriting it. The [gl-state] tripwire still detects
+        // any future leak.
+        _gl.DepthMask(true);
         _gl!.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit | ClearBufferMask.StencilBufferBit);
 
         // WB GameScene.cs:830-843 establishes CW as the frame-global