feat(render): Phase A8 — portal_stencil vert/frag shaders

Minimal pair for the indoor-cell stencil pipeline (#78). Vert transforms
world-space portal polygon vertices through uViewProjection; includes a
near-zero pos.w guard for coplanar-camera robustness (matches WB pattern).
Frag either passes through gl_FragCoord.z or writes gl_FragDepth=1.0
based on uWriteFarDepth; FragColor declared but suppressed via ColorMask
on the CPU side.

Matches WorldBuilder's PortalStencil.vert/.frag at
references/WorldBuilder/Chorizite.OpenGLSDLBackend/Shaders/.
Uses #version 430 core consistent with acdream's mesh_modern shaders.
Deployed to bin/ via existing Rendering\Shaders\*.* .csproj glob.

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

src/AcDream.App/Rendering/Shaders/portal_stencil.frag

@@ -0,0 +1,34 @@
+#version 430 core
+//
+// Phase A8 — portal stencil mark + far-depth punch.
+//
+// uWriteFarDepth = 0  → pass through gl_FragCoord.z (used for the
+//                       stencil mark pass; depth mask is off anyway).
+// uWriteFarDepth != 0 → write gl_FragDepth = 1.0 (the far-depth punch
+//                       pass; depth mask is on, color is off).
+//
+// Matches WorldBuilder's PortalStencil.frag at
+// references/WorldBuilder/Chorizite.OpenGLSDLBackend/Shaders/PortalStencil.frag
+
+out vec4 FragColor;
+
+uniform int uWriteFarDepth;
+
+void main()
+{
+    if (uWriteFarDepth != 0)
+    {
+        // Write far depth to clear the depth buffer in the portal region.
+        // This punches through the building exterior's depth so interior
+        // geometry at any depth can be rendered through the stencil mask.
+        gl_FragDepth = 1.0;
+    }
+    else
+    {
+        gl_FragDepth = gl_FragCoord.z;
+    }
+
+    // Color writes are suppressed via ColorMask(false) on the CPU side.
+    // Output is required by GLSL but will not be written to the framebuffer.
+    FragColor = vec4(0.0);
+}

src/AcDream.App/Rendering/Shaders/portal_stencil.vert

@@ -0,0 +1,25 @@
+#version 430 core
+//
+// Phase A8 — portal stencil mark + far-depth punch.
+//
+// Position is in WORLD space (pipeline transforms cell-local portal
+// polygon vertices through cell.WorldTransform on the CPU before
+// uploading to the VBO). Output is clip space via uViewProjection.
+
+layout(location = 0) in vec3 aPosition;
+
+uniform mat4 uViewProjection;
+
+void main()
+{
+    vec4 pos = uViewProjection * vec4(aPosition, 1.0);
+
+    // Prevent near-zero clipping issues when the camera is perfectly
+    // coplanar with the portal polygon.
+    if (abs(pos.w) < 0.001)
+    {
+        pos.w = pos.w < 0.0 ? -0.001 : 0.001;
+    }
+
+    gl_Position = pos;
+}