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/model/object.rs

@@ -0,0 +1,151 @@
+// Shared value types used by the dispatch and grip systems.
+
+use acadrust::types::{Color as AcadColor, LineWeight};
+use glam::Vec3;
+
+/// The kind of value held by a property row.
+#[derive(Clone, Debug, PartialEq)]
+pub enum PropValue {
+    /// Read-only display text.
+    ReadOnly(String),
+    /// Editable numeric/text field.
+    EditText(String),
+    /// Layer name — rendered as a combo_box.
+    LayerChoice(String),
+    /// Generic string choice rendered as a combo_box.
+    Choice {
+        selected: String,
+        options: Vec<String>,
+    },
+    /// ACI/RGB/ByLayer/ByBlock color — rendered as a color picker.
+    ColorChoice(AcadColor),
+    /// Color varies across the current multi-selection.
+    ColorVaries,
+    /// Line weight — rendered as a combo_box.
+    LwChoice(LineWeight),
+    /// Lineweight varies across the current multi-selection.
+    LwVaries,
+    /// Linetype name — rendered as a combo_box.
+    LinetypeChoice(String),
+    /// Boolean flag — rendered as a toggle button (e.g. Invisible).
+    BoolToggle { field: &'static str, value: bool },
+    /// Hatch pattern name — rendered as a combo_box from the catalog.
+    HatchPatternChoice(String),
+}
+
+/// A single property row in the Properties panel.
+#[derive(Clone, Debug, PartialEq)]
+pub struct Property {
+    pub label: String,
+    /// Stable field identifier used in `PropGeomInput` / `PropGeomCommit` messages.
+    pub field: &'static str,
+    pub value: PropValue,
+}
+
+/// A named section of properties (e.g. "General", "Geometry").
+#[derive(Clone, Debug, PartialEq)]
+pub struct PropSection {
+    pub title: String,
+    pub props: Vec<Property>,
+}
+
+// ── Grip types ─────────────────────────────────────────────────────────────
+
+/// Visual marker shape for a grip point. The complete vocabulary that
+/// matches the standard CAD grip conventions:
+/// * `Square` — endpoint / vertex / centre. Drag → moves a single
+///   point or translates the entity.
+/// * `Rectangle` — direction-aware mid-segment stretch handle.
+///   Drawn as a small box rotated along `dir` (the in-plane segment
+///   direction in world XY). Used for polyline / wipeout / image /
+///   dimension segment midpoints.
+/// * `Triangle` — directional indicator (Phase 2: dynamic-block
+///   parameters, dimension reverse-arrow flips, multi-functional
+///   hover popups).
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[allow(dead_code)]
+pub enum GripShape {
+    Square,
+    Rectangle,
+    Triangle,
+}
+
+/// Describes one grip point for an entity.
+#[derive(Clone, Debug)]
+pub struct GripDef {
+    /// Object-local identifier (stable index, unique per object instance).
+    pub id: usize,
+    /// World-space position of the grip, in f64. Entity coordinates can sit at
+    /// UTM magnitudes (1e7); casting to f32 before the world-offset subtraction
+    /// loses ~1 drawing unit and draws the grip visibly off the wire. Producers
+    /// fill this straight from the f64 entity data; the offset is subtracted in
+    /// f64 and only then cast for screen-space math.
+    pub world: glam::DVec3,
+    /// `true` → midpoint / centre grip (drags the whole shape).
+    /// `false` → endpoint / vertex grip (moves a single point).
+    pub is_midpoint: bool,
+    /// Visual marker shape for the grip.
+    pub shape: GripShape,
+    /// World-XY direction vector used to orient a `Rectangle` grip
+    /// along its segment. `None` for shapes that don't need rotation
+    /// (Square, Triangle in non-directional contexts).
+    pub dir: Option<[f32; 2]>,
+}
+
+/// How to apply a grip drag result.
+#[derive(Clone, Debug)]
+pub enum GripApply {
+    /// Move a specific vertex to this absolute world position.
+    Absolute(Vec3),
+    /// Translate the whole object by this delta vector.
+    Translate(Vec3),
+}
+
+/// One entry in the hover-popup menu that opens when the cursor dwells
+/// on a grip. The `label` is the user-visible string; `action` is the
+/// operation the entity will perform when the item is committed.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct GripMenuItem {
+    pub label: &'static str,
+    pub action: GripMenuAction,
+}
+
+/// All operations a grip popup menu can dispatch. Entity-specific code
+/// in `apply_grip_menu` decodes these into edits. `Stretch` is the
+/// default no-op-equivalent — picking it just starts the regular
+/// stretch drag, identical to clicking the grip with no popup open.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[allow(dead_code)]
+pub enum GripMenuAction {
+    Stretch,
+    Lengthen,
+    Radius,
+    ArcLength,
+    AddVertex,
+    RemoveVertex,
+    ConvertToArc,
+    ConvertToLine,
+    StretchVertex,
+    AddLeader,
+    RemoveLeader,
+    ReverseArrows,
+    MoveWithDimLine,
+    MoveWithLeader,
+    MoveIndependent,
+    ResetText,
+    RotateText,
+    AboveDimLine,
+    Center,
+    OriginPoint,
+    HatchAngle,
+    HatchScale,
+    HatchPattern,
+    TangentDirection,
+    AddFitPoint,
+    RemoveFitPoint,
+    Refit,
+    RefineVertices,
+    MoveWithText,
+    StackText,
+    UnstackText,
+}