feat(render): Phase G.1 — billboard particle renderer for weather + spells

Add ParticleRenderer that draws every live particle from the shared
ParticleSystem as a billboarded quad. Unit quad VBO + per-instance
(pos, size, color) VBO with glVertexAttribDivisor for one draw call
per emitter. Billboards using the camera's basis vectors so quads
always face the viewer.

Fragment shader does a procedural radial falloff (no texture pipeline
needed — raindrops / snowflakes read as soft dots). AttachLocal
emitters get re-centred on the camera each frame so the rain volume
follows the player per r12 §7.

Two-pass render splits additive from alpha-blend emitters so blend
state flips once per kind rather than per-emitter.

Wired into GameWindow.OnRender after static-mesh draw with depth
write off (particles occluded by walls but don't self-occlude).

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -140,6 +140,7 @@ public sealed class GameWindow : IDisposable
     private readonly AcDream.Core.Vfx.EmitterDescRegistry _emitterRegistry = new();
     private AcDream.Core.Vfx.ParticleSystem? _particleSystem;
     private AcDream.Core.Vfx.ParticleHookSink? _particleSink;
+    private AcDream.App.Rendering.ParticleRenderer? _particleRenderer;
 
     // Remote-entity motion inference: tracks when each remote entity last
     // moved meaningfully. Used in TickAnimations to swap to Ready when
@@ -737,6 +738,12 @@ public sealed class GameWindow : IDisposable
         _skyRenderer = new AcDream.App.Rendering.Sky.SkyRenderer(
             _gl, _dats, skyShader, _textureCache);
 
+        // Phase G.1 particle renderer — renders rain / snow / spell auras
+        // spawned into the shared ParticleSystem as billboard quads.
+        // Weather uses AttachLocal emitters so the rain volume follows
+        // the player.
+        _particleRenderer = new ParticleRenderer(_gl, shadersDir);
+
         // Phase A.1: replace the one-shot 3×3 preload with a streaming controller.
         // Parse runtime radius from environment (default 2 → 5×5 window).
         // Values outside [0, 8] fall back to the field default of 2.
@@ -3035,6 +3042,13 @@ public sealed class GameWindow : IDisposable
                 neverCullLandblockId: playerLb,
                 visibleCellIds: visibility?.VisibleCellIds);
 
+            // Phase G.1 / E.3: draw all live particles after opaque
+            // scene geometry so alpha blending composites correctly.
+            // Runs with depth test on (particles occluded by walls)
+            // but depth write off (no self-occlusion sorting needed).
+            if (_particleSystem is not null && _particleRenderer is not null)
+                _particleRenderer.Draw(_particleSystem, camera, camPos);
+
             // Debug: draw collision shapes as wireframe cylinders around the
             // player so we can visually verify alignment with scenery meshes.
             if (_debugCollisionVisible && _debugLines is not null)
@@ -3690,6 +3704,7 @@ public sealed class GameWindow : IDisposable
         _shader?.Dispose();
         _sceneLightingUbo?.Dispose();
         _skyRenderer?.Dispose();
+        _particleRenderer?.Dispose();
         _debugLines?.Dispose();
         _textRenderer?.Dispose();
         _debugFont?.Dispose();