feat: implement M2 ribbon emitter rendering for spell trail effects

Parse M2RibbonEmitter data (WotLK format) from M2 files — bone index,
position, color/alpha/height tracks, edgesPerSecond, edgeLifetime,
gravity. Add CPU-side trail simulation per instance (edge birth at bone
world position, lifetime expiry, gravity droop). New m2_ribbon.vert/frag
shaders render a triangle-strip quad per emitter using the existing
particleTexLayout_ descriptor set. Supports both alpha-blend and additive
pipeline variants based on material blend mode. Fixes invisible spell
trail effects (~5-10%% of spell visuals) that were silently skipped.

assets/shaders/m2_ribbon.frag.glsl

@@ -0,0 +1,25 @@
+#version 450
+
+// M2 ribbon emitter fragment shader.
+// Samples the ribbon texture, multiplied by vertex color and alpha.
+// Uses additive blending (pipeline-level) for magic/spell trails.
+
+layout(set = 1, binding = 0) uniform sampler2D uTexture;
+
+layout(location = 0) in vec3 vColor;
+layout(location = 1) in float vAlpha;
+layout(location = 2) in vec2 vUV;
+layout(location = 3) in float vFogFactor;
+
+layout(location = 0) out vec4 outColor;
+
+void main() {
+    vec4 tex = texture(uTexture, vUV);
+    // For additive ribbons alpha comes from texture luminance; multiply by vertex alpha.
+    float a = tex.a * vAlpha;
+    if (a < 0.01) discard;
+    vec3 rgb = tex.rgb * vColor;
+    // Ribbons fade slightly with fog (additive blend attenuated toward black = invisible in fog).
+    rgb *= vFogFactor;
+    outColor = vec4(rgb, a);
+}

assets/shaders/m2_ribbon.vert.glsl

@@ -0,0 +1,43 @@
+#version 450
+
+// M2 ribbon emitter vertex shader.
+// Ribbon geometry is generated CPU-side as a triangle strip.
+// Vertex format: pos(3) + color(3) + alpha(1) + uv(2) = 9 floats.
+
+layout(set = 0, binding = 0) uniform PerFrame {
+    mat4 view;
+    mat4 projection;
+    mat4 lightSpaceMatrix;
+    vec4 lightDir;
+    vec4 lightColor;
+    vec4 ambientColor;
+    vec4 viewPos;
+    vec4 fogColor;
+    vec4 fogParams;
+    vec4 shadowParams;
+};
+
+layout(location = 0) in vec3 aPos;
+layout(location = 1) in vec3 aColor;
+layout(location = 2) in float aAlpha;
+layout(location = 3) in vec2 aUV;
+
+layout(location = 0) out vec3 vColor;
+layout(location = 1) out float vAlpha;
+layout(location = 2) out vec2 vUV;
+layout(location = 3) out float vFogFactor;
+
+void main() {
+    vec4 worldPos = vec4(aPos, 1.0);
+    vec4 viewPos4  = view * worldPos;
+    gl_Position    = projection * viewPos4;
+
+    float dist      = length(viewPos4.xyz);
+    float fogStart  = fogParams.x;
+    float fogEnd    = fogParams.y;
+    vFogFactor      = clamp((fogEnd - dist) / max(fogEnd - fogStart, 0.001), 0.0, 1.0);
+
+    vColor = aColor;
+    vAlpha = aAlpha;
+    vUV    = aUV;
+}