feat: raster image render pipeline (IMAGE command + GPU textured quad)

- Add IMAGE / IMAGEATTACH / IM command with async file-picker (rfd)
- src/scene/image_model.rs: ImageModel with RGBA pixel data + quad corners
- src/scene/pipeline/image_gpu.rs: ImageGpu with wgpu texture, sampler, opacity uniform
- src/scene/pipeline/mod.rs: image render pipeline (pass 2, between hatches and meshes)
- src/scene/render.rs: include images in Primitive, call upload_images in prepare
- src/scene/mod.rs: images HashMap, populate_images_from_document, cache on add_entity
- src/modules/home/draw/raster_image.rs: ImageCommand (two-click: origin + width, aspect-correct preview)
- src/io/mod.rs: pick_image_file() async helper (file dialog + dimension decode)
- app: ImagePick / ImagePickResult messages, populate on file open and undo/redo

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/shaders/image.wgsl

@@ -0,0 +1,53 @@
+// Textured-quad shader for raster images (RasterImage entity).
+// Renders a four-vertex quad (two triangles) with a sampled texture.
+
+// ── Bind group 0: shared projection uniforms ─────────────────────────────────
+struct Uniforms {
+    mvp:    mat4x4<f32>,
+    view:   mat4x4<f32>,
+    width:  f32,
+    height: f32,
+    cam_z:  f32,
+    _pad:   f32,
+};
+
+@group(0) @binding(0) var<uniform> u: Uniforms;
+
+// ── Bind group 1: per-image texture + sampler ────────────────────────────────
+@group(1) @binding(0) var img_texture: texture_2d<f32>;
+@group(1) @binding(1) var img_sampler: sampler;
+
+// Per-image params (fade, clip flag, etc.)
+struct ImageParams {
+    opacity: f32,
+    _pad0: f32,
+    _pad1: f32,
+    _pad2: f32,
+};
+@group(1) @binding(2) var<uniform> img_params: ImageParams;
+
+// ── Vertex stage ──────────────────────────────────────────────────────────────
+struct VertIn {
+    @location(0) pos:  vec3<f32>,
+    @location(1) uv:   vec2<f32>,
+};
+
+struct VertOut {
+    @builtin(position) clip_pos: vec4<f32>,
+    @location(0)       uv:       vec2<f32>,
+};
+
+@vertex
+fn vs_main(in: VertIn) -> VertOut {
+    var out: VertOut;
+    out.clip_pos = u.mvp * vec4<f32>(in.pos, 1.0);
+    out.uv = in.uv;
+    return out;
+}
+
+// ── Fragment stage ────────────────────────────────────────────────────────────
+@fragment
+fn fs_main(in: VertOut) -> @location(0) vec4<f32> {
+    let col = textureSample(img_texture, img_sampler, in.uv);
+    return vec4<f32>(col.rgb, col.a * img_params.opacity);
+}