feat(vfx): Phase C.1 — PES particle renderer + post-review fixes

Ports retail's ParticleEmitterInfo / Particle::Init / Particle::Update
(0x005170d0..0x0051d400) and PhysicsScript runtime to a C# data-layer
plus a Silk.NET billboard renderer. Sky-PES path is debug-only behind
ACDREAM_ENABLE_SKY_PES because named-retail decomp confirms GameSky
copies SkyObject.pes_id but never reads it (CreateDeletePhysicsObjects
0x005073c0, MakeObject 0x00506ee0, UseTime 0x005075b0).

Post-review fixes folded into this commit:

H1: AttachLocal (is_parent_local=1) follows live parent each frame.
    ParticleSystem.UpdateEmitterAnchor + ParticleHookSink.UpdateEntityAnchor
    let the owning subsystem refresh AnchorPos every tick — matches
    ParticleEmitter::UpdateParticles 0x0051d2d4 which re-reads the live
    parent frame when is_parent_local != 0. Drops the renderer-side
    cameraOffset hack that only worked when the parent was the camera.

H3: Strip the long stale comment in GfxObjMesh.cs that contradicted the
    retail-faithful (1 - translucency) opacity formula. The code was
    right; the comment was a leftover from an earlier hypothesis and
    would have invited a wrong "fix".

M1: SkyRenderer tracks textures whose wrap mode it set to ClampToEdge
    and restores them to Repeat at end-of-pass, so non-sky renderers
    that share the GL handle can't silently inherit clamped wrap state.

M2: Post-scene Z-offset (-120m) only fires when the SkyObject is
    weather-flagged AND bit 0x08 is clear, matching retail
    GameSky::UpdatePosition 0x00506dd0. The old code applied it to
    every post-scene object — a no-op today (every Dereth post-scene
    entry happens to be weather-flagged) but a future post-scene-only
    sun rim would have been pushed below the camera.

M4: ParticleSystem.EmitterDied event lets ParticleHookSink prune dead
    handles from the per-entity tracking dictionaries, fixing a slow
    leak where naturally-expired emitters' handles stayed in the
    ConcurrentBag forever during long sessions.

M5: SkyPesEntityId moves the post-scene flag bit to 0x08000000 so it
    can't ever overlap the object-index range. Synthetic IDs stay in
    the reserved 0xFxxxxxxx space.

New tests (ParticleSystemTests + ParticleHookSinkTests):
- UpdateEmitterAnchor_AttachLocal_ParticlePositionFollowsLiveAnchor
- UpdateEmitterAnchor_AttachLocalCleared_ParticleFrozenAtSpawnOrigin
- EmitterDied_FiresOncePerHandle_AfterAllParticlesExpire
- Birthrate_PerSec_EmitsOnePerTickWhenIntervalElapsed (retail-faithful
  single-emit-per-frame behavior)
- UpdateEntityAnchor_WithAttachLocal_MovesParticleToLiveAnchor
- EmitterDied_PrunesPerEntityHandleTracking

dotnet build green, dotnet test green: 695 / 393 / 243 = 1331 passed
(up from 1325).

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

src/AcDream.Core/Textures/SurfaceDecoder.cs

@@ -37,9 +37,9 @@ public static class SurfaceDecoder
                 PixelFormat.PFID_R8G8B8 => DecodeR8G8B8(rs),
                 PixelFormat.PFID_A8R8G8B8 => DecodeA8R8G8B8(rs),
                 PixelFormat.PFID_X8R8G8B8 => DecodeX8R8G8B8(rs),
-                PixelFormat.PFID_DXT1 => DecodeBc(rs, CompressionFormat.Bc1),
-                PixelFormat.PFID_DXT3 => DecodeBc(rs, CompressionFormat.Bc2),
-                PixelFormat.PFID_DXT5 => DecodeBc(rs, CompressionFormat.Bc3),
+                PixelFormat.PFID_DXT1 => DecodeBc(rs, CompressionFormat.Bc1, isClipMap),
+                PixelFormat.PFID_DXT3 => DecodeBc(rs, CompressionFormat.Bc2, isClipMap),
+                PixelFormat.PFID_DXT5 => DecodeBc(rs, CompressionFormat.Bc3, isClipMap),
                 PixelFormat.PFID_A8 or PixelFormat.PFID_CUSTOM_LSCAPE_ALPHA => DecodeA8(rs),
                 PixelFormat.PFID_P8 when palette is not null => DecodeP8(rs, palette, isClipMap),
                 PixelFormat.PFID_INDEX16 when palette is not null => DecodeIndex16(rs, palette, isClipMap),
@@ -245,7 +245,7 @@ public static class SurfaceDecoder
         return new DecodedTexture(rgba, rs.Width, rs.Height);
     }
 
-    private static DecodedTexture DecodeBc(RenderSurface rs, CompressionFormat format)
+    private static DecodedTexture DecodeBc(RenderSurface rs, CompressionFormat format, bool isClipMap)
     {
         var pixels = BcDecoder.DecodeRaw(rs.SourceData, rs.Width, rs.Height, format);
         var rgba = new byte[rs.Width * rs.Height * 4];
@@ -256,6 +256,8 @@ public static class SurfaceDecoder
             rgba[s + 1] = pixels[i].g;
             rgba[s + 2] = pixels[i].b;
             rgba[s + 3] = pixels[i].a;
+            if (isClipMap && rgba[s + 0] == 0 && rgba[s + 1] == 0 && rgba[s + 2] == 0)
+                rgba[s + 3] = 0;
         }
         return new DecodedTexture(rgba, rs.Width, rs.Height);
     }