feat(#188): fading-wall + sliding-door translucency; hold open past animation settle

Lands the fading-secret-door feature and fixes the door "flip-back" that
surfaced while testing it.

#188 — fading-wall doors (e.g. "Pedestal Weak Spot") fade their wall part
out via TransparentPartHook instead of swinging:
  - TranslucencyHookSink consumes TransparentPartHook -> TranslucencyFadeManager
    (per-(entity,part) linear translucency ramp; holds at End frame).
  - WbDrawDispatcher: new per-instance alpha SSBO (binding 7); ClassifyBatches
    takes opacityMultiplier (1 - translucency, per CMaterial::SetTranslucencySimple
    0x005396f0) forcing AlphaBlend; fully-invisible parts skipped.
  - mesh_modern.vert/.frag: binding-7 InstanceAlphaBuf -> vOpacityMultiplier ->
    FragColor.a *= vOpacityMultiplier.
  - Register AP-89: the fade multiplies sampled texture alpha, not a separate
    D3D9 material alpha channel (observably identical for texture-alpha==1 surfaces).

Door flip-back fix (affected BOTH #188 fading walls AND #187 sliding doors): a
door/wall that finished opening holds a single unchanging frame, so the
uncommitted IsEntityCurrentlyMoving cache-bypass narrowing dropped it onto the
Tier-1 static cache -- which only remembers the REST pose + opacity 1.0 --
snapping it visually shut/opaque while physics stayed open. Reverted that
narrowing: every Sequencer entity stays on the per-frame path (live pose + live
fade opacity), the known-good pre-optimization behavior. The per-frame CPU cost
that narrowing chased was a Debug-build artifact -- Release is GPU-bound
(~200 fps in Sawato, measured), so the unconditional add is free where it
matters. Left a code comment barring re-introduction.

Tests: full Core suite green (2649 passed, 2 skipped). Live visual gate PASSED --
both fading-wall and sliding doors hold open.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/AcDream.Core.Tests/Rendering/TranslucencyHookSinkTests.cs

@@ -0,0 +1,70 @@
+using System.Numerics;
+using AcDream.Core.Rendering;
+using DatReaderWriter.Types;
+using Xunit;
+
+namespace AcDream.Core.Tests.Rendering;
+
+public sealed class TranslucencyHookSinkTests
+{
+    [Fact]
+    public void TransparentPartHook_ForwardsFieldsVerbatimToManager()
+    {
+        var mgr = new TranslucencyFadeManager();
+        var sink = new TranslucencyHookSink(mgr);
+
+        var hook = new TransparentPartHook { PartIndex = 3, Start = 0.1f, End = 0.9f, Time = 0.5f };
+        sink.OnHook(entityId: 42, entityWorldPosition: Vector3.Zero, hook: hook);
+
+        Assert.True(mgr.TryGetCurrentValue(42, 3, out float value));
+        Assert.Equal(0.1f, value); // t=0 value, readable immediately after the hook fires
+    }
+
+    [Fact]
+    public void TransparentPartHook_InstantTime_CommitsEndImmediately()
+    {
+        var mgr = new TranslucencyFadeManager();
+        var sink = new TranslucencyHookSink(mgr);
+
+        var hook = new TransparentPartHook { PartIndex = 0, Start = 0f, End = 1f, Time = 0f };
+        sink.OnHook(entityId: 1, entityWorldPosition: Vector3.Zero, hook: hook);
+
+        Assert.True(mgr.TryGetCurrentValue(1, 0, out float value));
+        Assert.Equal(1f, value);
+    }
+
+    [Fact]
+    public void EtherealHook_IsIgnored()
+    {
+        var mgr = new TranslucencyFadeManager();
+        var sink = new TranslucencyHookSink(mgr);
+
+        sink.OnHook(entityId: 1, entityWorldPosition: Vector3.Zero, hook: new EtherealHook { Ethereal = true });
+
+        Assert.False(mgr.TryGetCurrentValue(1, 0, out _));
+    }
+
+    [Fact]
+    public void WholeObjectTransparentHook_IsIgnored()
+    {
+        var mgr = new TranslucencyFadeManager();
+        var sink = new TranslucencyHookSink(mgr);
+
+        // #188 scope: the whole-object variant is deliberately NOT ported this pass.
+        sink.OnHook(entityId: 1, entityWorldPosition: Vector3.Zero,
+            hook: new TransparentHook { Start = 0f, End = 1f, Time = 1f });
+
+        Assert.False(mgr.TryGetCurrentValue(1, 0, out _));
+    }
+
+    [Fact]
+    public void UnrelatedHook_IsIgnored()
+    {
+        var mgr = new TranslucencyFadeManager();
+        var sink = new TranslucencyHookSink(mgr);
+
+        sink.OnHook(entityId: 1, entityWorldPosition: Vector3.Zero, hook: new SoundTableHook());
+
+        Assert.False(mgr.TryGetCurrentValue(1, 0, out _));
+    }
+}