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/Shaders/particle.frag

@@ -0,0 +1,18 @@
+#version 430 core
+
+in vec2 vTex;
+in vec4 vColor;
+out vec4 fragColor;
+
+// Procedural rain/snow streak — no texture, just a radial falloff
+// centred on the quad so droplets read as small soft circles. Good
+// enough for weather + basic spell auras without a texture pipeline.
+
+void main() {
+    // Signed distance from quad center (in UV space).
+    vec2 d = vTex - vec2(0.5, 0.5);
+    float r = length(d) * 2.0;      // 0 at center, 1 at corner
+    float falloff = smoothstep(1.0, 0.4, r);
+    if (falloff < 0.02) discard;
+    fragColor = vec4(vColor.rgb, vColor.a * falloff);
+}

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

@@ -0,0 +1,31 @@
+#version 430 core
+
+// Unit quad vertex (XY -0.5..+0.5)
+layout(location = 0) in vec2 aQuad;
+layout(location = 1) in vec2 aTex;
+
+// Per-instance: world-space center + size
+layout(location = 2) in vec4 aPosAndSize;
+layout(location = 3) in vec4 aColor;
+
+uniform mat4 uViewProjection;
+uniform vec3 uCameraRight;
+uniform vec3 uCameraUp;
+
+out vec2 vTex;
+out vec4 vColor;
+
+void main() {
+    vec3 center = aPosAndSize.xyz;
+    float size = aPosAndSize.w;
+
+    // Billboard: offset the quad vertex along the camera's right + up
+    // basis vectors so it always faces the viewer.
+    vec3 world = center
+               + uCameraRight * (aQuad.x * size)
+               + uCameraUp    * (aQuad.y * size);
+
+    vTex = aTex;
+    vColor = aColor;
+    gl_Position = uViewProjection * vec4(world, 1.0);
+}