Point clouds: drawn from their scans, placed, cropped and attached as the reference does

Scans (.rcs) and projects (.rcp) are read and their points drawn as
screen-sized dots, coloured by the cloud's stylization. The cloud's
placement carries its scale and rotation; its extents and crops stay in
the scan's coordinates. A missing scan shows the extents with the saved
path. -POINTCLOUDATTACH / POINTCLOUDATTACH, POINTCLOUDCROP and the
POINTCLOUD* settings follow the reference's prompts and messages, the
Properties list is the reference's, and a locked cloud is left alone by
edits.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/shaders/point_cloud.wgsl

@@ -0,0 +1,63 @@
+// Point cloud sprites: one instance per point, drawn as a square of
+// POINTCLOUDPOINTSIZE pixels around its projected position, depth-tested.
+
+// ── Bind group 0: shared projection uniforms ─────────────────────────────────
+// Must match the leading fields of `Uniforms` (scene::pipeline::uniforms).
+struct Uniforms {
+    viewport_size:      vec2<f32>,
+    world_per_pixel:    f32,
+    lwdisplay_enable:   f32,
+    flat_shade:         f32,
+    transparency_enable: f32,
+    _pad:               vec2<f32>,
+    // Relative-to-eye (double-single): see wire.wgsl.
+    view_rot:           mat4x4<f32>,
+    eye_high:           vec3<f32>,
+    _pad_eh:            f32,
+    eye_low:            vec3<f32>,
+    _pad_el:            f32,
+};
+
+@group(0) @binding(0) var<uniform> u: Uniforms;
+
+struct PointParams {
+    size_px: f32,
+    _pad0: f32,
+    _pad1: f32,
+    _pad2: f32,
+};
+@group(1) @binding(0) var<uniform> params: PointParams;
+
+struct PointIn {
+    @location(0) pos:     vec3<f32>,
+    @location(1) pos_low: vec3<f32>,
+    @location(2) color:   vec4<f32>,
+};
+
+struct VertOut {
+    @builtin(position) clip_pos: vec4<f32>,
+    @location(0)       color:    vec4<f32>,
+};
+
+@vertex
+fn vs_main(@builtin(vertex_index) corner: u32, in: PointIn) -> VertOut {
+    var out: VertOut;
+    let rel = (in.pos - u.eye_high) + (in.pos_low - u.eye_low);
+    let clip = u.view_rot * vec4<f32>(rel, 1.0);
+    // Two triangles covering the square: corners in -1..1.
+    var corners = array<vec2<f32>, 6>(
+        vec2<f32>(-1.0, -1.0), vec2<f32>(1.0, -1.0), vec2<f32>(1.0, 1.0),
+        vec2<f32>(-1.0, -1.0), vec2<f32>(1.0, 1.0), vec2<f32>(-1.0, 1.0),
+    );
+    // Half the size in pixels → NDC (2 / viewport), scaled by w to stay in
+    // clip space.
+    let half_ndc = params.size_px / max(u.viewport_size, vec2<f32>(1.0, 1.0));
+    out.clip_pos = vec4<f32>(clip.xy + corners[corner] * half_ndc * clip.w, clip.z, clip.w);
+    out.color = in.color;
+    return out;
+}
+
+@fragment
+fn fs_main(in: VertOut) -> @location(0) vec4<f32> {
+    return in.color;
+}