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/mod.rs

@@ -1,5 +1,5 @@
 pub mod hit_test;
-pub mod selection;
+pub mod selection_state;
 pub mod grip;
 pub mod quadtree;
 pub mod xclip;