refactor(scene): group modules into role-based subfolders

Move the 30 flat modules under src/scene/ into six folders by role:
convert (entity→geometry), text (fonts/shaping), model (render models),
pick (hit-test/selection/grips/xclip), view (camera/transform/render),
cache (block + property caches). pipeline/ is unchanged.

Full path migration — all `scene::X` references become
`scene::<group>::X`, no compatibility re-exports. render.rs items that
were `pub(super)` (parent was scene) become `pub(in crate::scene)` to
keep their original scene-wide visibility.

No behavior change; build and tests are green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/pick/grip.rs

@@ -0,0 +1,130 @@
+//! OpenCADStudio-style grip editing.
+
+use acadrust::Handle;
+use glam::{Mat4, Vec3};
+use iced::{Point, Rectangle};
+
+use crate::scene::model::object::{GripDef, GripShape};
+
+/// Pixel radius for grip hit-detection.
+pub const GRIP_THRESHOLD_PX: f32 = 8.0;
+/// Half-size of the rendered grip square / diamond in pixels.
+pub const GRIP_HALF_PX: f32 = 5.0;
+
+// ── Active drag state ─────────────────────────────────────────────────────
+
+/// Stored on `OpenCADStudio` while a grip is being dragged.
+#[derive(Clone, Debug)]
+pub struct GripEdit {
+    /// Handle of the entity being edited.
+    pub handle: Handle,
+    /// Index into the entity's grip list.
+    pub grip_id: usize,
+    /// `true` → midpoint / translate grip; `false` → endpoint / absolute grip.
+    pub is_translate: bool,
+    /// World-space position of the grip when the drag started (ortho/polar base).
+    pub origin_world: Vec3,
+    /// Last world-space cursor position (needed for incremental delta on translate drags).
+    pub last_world: Vec3,
+}
+
+// ── Screen-space helpers ───────────────────────────────────────────────────
+
+/// Project a slice of `GripDef`s to screen space.
+/// Returns `(grip_id, screen_pos, is_midpoint, shape)` for each grip.
+pub fn grips_to_screen(
+    grips: &[GripDef],
+    view_proj: Mat4,
+    bounds: Rectangle,
+) -> Vec<(usize, Point, bool, GripShape, Option<[f32; 2]>)> {
+    grips
+        .iter()
+        .map(|g| {
+            let ndc = view_proj.project_point3(g.world.as_vec3());
+            let screen = Point::new(
+                bounds.x + (ndc.x + 1.0) * 0.5 * bounds.width,
+                bounds.y + (1.0 - ndc.y) * 0.5 * bounds.height,
+            );
+            (g.id, screen, g.is_midpoint, g.shape, g.dir)
+        })
+        .collect()
+}
+
+/// Paper-space variant: project grips using the 2-D linear `to_px` transform.
+/// Parameters match the `to_px` closure in `paper_canvas.rs`.
+pub fn grips_to_screen_paper(
+    grips: &[GripDef],
+    tx: f32,
+    ty: f32,
+    half_w: f32,
+    half_h: f32,
+    bounds: Rectangle,
+) -> Vec<(usize, Point, bool, GripShape, Option<[f32; 2]>)> {
+    grips
+        .iter()
+        .map(|g| {
+            let screen = Point::new(
+                (g.world.x as f32 - tx + half_w) / (2.0 * half_w) * bounds.width,
+                (ty + half_h - g.world.y as f32) / (2.0 * half_h) * bounds.height,
+            );
+            (g.id, screen, g.is_midpoint, g.shape, g.dir)
+        })
+        .collect()
+}
+
+/// Paper-space hit-test variant (mirrors `find_hit_grip` but uses 2-D projection).
+pub fn find_hit_grip_paper(
+    cursor: Point,
+    grips: &[GripDef],
+    tx: f32,
+    ty: f32,
+    half_w: f32,
+    half_h: f32,
+    bounds: Rectangle,
+) -> Option<(usize, bool, Vec3)> {
+    let mut best_dist = GRIP_THRESHOLD_PX;
+    let mut best: Option<(usize, bool, Vec3)> = None;
+
+    for g in grips {
+        let screen = Point::new(
+            (g.world.x as f32 - tx + half_w) / (2.0 * half_w) * bounds.width,
+            (ty + half_h - g.world.y as f32) / (2.0 * half_h) * bounds.height,
+        );
+        let dx = screen.x - cursor.x;
+        let dy = screen.y - cursor.y;
+        let d = (dx * dx + dy * dy).sqrt();
+        if d < best_dist {
+            best_dist = d;
+            best = Some((g.id, g.is_midpoint, g.world.as_vec3()));
+        }
+    }
+    best
+}
+
+/// Find the closest grip within `GRIP_THRESHOLD_PX` pixels of `cursor`.
+/// Returns `(grip_id, is_translate, world_pos)` if found, else `None`.
+pub fn find_hit_grip(
+    cursor: Point,
+    grips: &[GripDef],
+    view_proj: Mat4,
+    bounds: Rectangle,
+) -> Option<(usize, bool, Vec3)> {
+    let mut best_dist = GRIP_THRESHOLD_PX;
+    let mut best: Option<(usize, bool, Vec3)> = None;
+
+    for g in grips {
+        let ndc = view_proj.project_point3(g.world.as_vec3());
+        let screen = Point::new(
+            (ndc.x + 1.0) * 0.5 * bounds.width,
+            (1.0 - ndc.y) * 0.5 * bounds.height,
+        );
+        let dx = screen.x - cursor.x;
+        let dy = screen.y - cursor.y;
+        let d = (dx * dx + dy * dy).sqrt();
+        if d < best_dist {
+            best_dist = d;
+            best = Some((g.id, g.is_midpoint, g.world.as_vec3()));
+        }
+    }
+    best
+}