refactor: rename mislabeled scene files to match contents

Two names collided with the pick subsystem and misdescribed their files:

- scene/hittest.rs -> scene/camera_ops.rs. Its contents are camera/view
  operations (zoom, fit-all, named-view restore, camera<->document sync,
  per-tile grid/snap), not hit-testing — the real screen-space hit-test
  lives in scene/pick/hit_test.rs.
- scene/pick/selection.rs -> scene/pick/selection_state.rs. It holds only
  the SelectionState struct; the name clashed with scene/selection.rs
  (the Scene selection operations).

Callers are unaffected (the crate::scene::SelectionState re-export path is
unchanged). No behavior change.

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

src/scene/pick/selection_state.rs

@@ -0,0 +1,32 @@
+use iced::time::Instant;
+use iced::Point;
+
+/// Mouse / selection interaction state for the viewport.
+#[derive(Clone, Default)]
+pub struct SelectionState {
+    pub vp_size: (f32, f32),
+    pub box_anchor: Option<Point>,
+    pub box_current: Option<Point>,
+    pub box_last: Option<(Point, Point)>,
+    pub box_crossing: bool,
+    pub box_last_crossing: bool,
+    pub poly_active: bool,
+    pub poly_points: Vec<Point>,
+    pub poly_crossing: bool,
+    pub poly_last_crossing: bool,
+    pub context_menu: Option<Point>,
+    /// True while the context menu's Draw Order sub-items are expanded.
+    pub draworder_submenu: bool,
+    pub last_move_pos: Option<Point>,
+    pub left_down: bool,
+    pub left_press_pos: Option<Point>,
+    pub left_press_time: Option<Instant>,
+    pub left_dragging: bool,
+    pub right_down: bool,
+    pub right_press_pos: Option<Point>,
+    pub right_dragging: bool,
+    pub right_last_pos: Option<Point>,
+    pub middle_down: bool,
+    pub middle_last_pos: Option<Point>,
+    pub middle_last_press_time: Option<Instant>,
+}