feat(nav): turntable orbit, selection/cursor pivot, Shift+MMB rotate (#229)

Rework 3D view navigation:

- Turntable / lock-horizon orbit: horizontal drag yaws about world +Z,
  vertical drag pitches about the horizontal camera-right axis, so the
  view never banks. Pitch is pole-clamped but the straight-down default
  view can still be tilted out of.
- Orbit centre: the selection's centre when something is selected,
  otherwise the point under the cursor — no longer only the file centre.
  The camera revolves rigidly about the pivot, so it stays put on screen
  and the view doesn't jump.
- Mouse scheme: Zoom = wheel, Pan = MMB, Rotate = Shift+MMB. Right-drag
  no longer orbits (right-click stays context menu / Enter). Floating
  viewports orbit with Shift+MMB too.

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

src/scene/pick/selection_state.rs

@@ -32,6 +32,10 @@ pub struct SelectionState {
     pub right_press_time: Option<Instant>,
     pub right_dragging: bool,
     pub right_last_pos: Option<Point>,
+    /// World point the current orbit drag revolves around (selection or model
+    /// centre), captured when the drag starts so it stays fixed for the whole
+    /// gesture. `None` when no orbit is in progress. (#229)
+    pub orbit_pivot: Option<glam::DVec3>,
     /// While a command is active, a right-click acts as Enter; the *next*
     /// consecutive right-click opens the context menu instead. This tracks
     /// whether the previous right-click already fired Enter. Reset by any