feat: lit particle shader for nebula dust weather effect

Replace NebulaDust's flat unshaded glow material with a custom
ShaderMaterial: a fake per-pixel puff normal on the billboarded quad
feeds a light() override so motes catch the directional light and a
nebula-core color bias, alpha gets a depth-texture soft-particle fade
so motes no longer hard-clip through boundary/decoration geometry,
and a per-particle hash adds subtle sparkle.

Game/shaders/nebula_dust.gdshader

@@ -0,0 +1,52 @@
+shader_type spatial;
+render_mode blend_mix, depth_draw_never, cull_disabled, specular_disabled;
+
+uniform sampler2D albedo_tex : source_color;
+uniform sampler2D depth_tex : hint_depth_texture, filter_linear_mipmap;
+uniform vec3 core_bias_color : source_color = vec3(1.0, 0.6, 0.85);
+uniform float core_bias_amount : hint_range(0.0, 1.0) = 0.35;
+uniform float emission_strength : hint_range(0.0, 5.0) = 2.5;
+uniform float soft_fade_distance : hint_range(0.0, 5.0) = 1.0;
+
+varying float v_flicker;
+
+void vertex() {
+	// true billboard: strip rotation from the per-instance modelview, keep translation + scale
+	mat4 mv = MODELVIEW_MATRIX;
+	mv[0].xyz = vec3(length(mv[0].xyz), 0.0, 0.0);
+	mv[1].xyz = vec3(0.0, length(mv[1].xyz), 0.0);
+	mv[2].xyz = vec3(0.0, 0.0, length(mv[2].xyz));
+	VERTEX = (mv * vec4(VERTEX, 1.0)).xyz;
+
+	// INSTANCE_CUSTOM.x is GPUParticles3D's per-particle random seed, baked at spawn
+	float seed = INSTANCE_CUSTOM.x;
+	float flicker_hash = fract(sin((seed + floor(TIME * 6.0)) * 127.1) * 43758.5453);
+	v_flicker = 0.85 + 0.15 * flicker_hash;
+}
+
+void fragment() {
+	// fake a puff/sphere normal from local UV so a camera-facing billboard can still catch directional light;
+	// the quad is billboarded to face the camera in vertex(), so this normal is already view-space aligned
+	vec2 centered = UV * 2.0 - 1.0;
+	float r2 = dot(centered, centered);
+	float mask = clamp(1.0 - r2, 0.0, 1.0);
+	NORMAL = normalize(vec3(centered, sqrt(max(mask, 0.001))));
+
+	vec4 tex = texture(albedo_tex, UV);
+	ALBEDO = tex.rgb * mix(vec3(1.0), core_bias_color, core_bias_amount);
+	EMISSION = core_bias_color * tex.rgb * emission_strength * 0.15;
+	ALPHA = tex.a * COLOR.a * mask;
+
+	float raw_depth = texture(depth_tex, SCREEN_UV).r;
+	vec3 ndc = vec3(SCREEN_UV * 2.0 - 1.0, raw_depth);
+	vec4 view_pos = INV_PROJECTION_MATRIX * vec4(ndc, 1.0);
+	view_pos.xyz /= view_pos.w;
+	float scene_depth = -view_pos.z;
+	float particle_depth = -VERTEX.z;
+	ALPHA *= clamp((scene_depth - particle_depth) / soft_fade_distance, 0.0, 1.0);
+}
+
+void light() {
+	float ndotl = clamp(dot(NORMAL, LIGHT), 0.0, 1.0);
+	DIFFUSE_LIGHT += LIGHT_COLOR * ndotl * ALBEDO * emission_strength * v_flicker * ATTENUATION;
+}