Home and Fit frame what is shown now; middle double-click goes Home (#701)

Home, Fit, the orbit pivot and the scene's depth range counted sketches the view doesn't draw (ones used by an extrude or revolve, ones in hidden components), so after a body moved away from its sketch Home framed the empty spot it came from. They now use the world-space bounds of what is shown now (view::visible_bounds / view::sketch_drawn, viewport::view_bounds). The pick-box cache holds its world state instead of its address. A middle-button double-click goes Home in every preset (Preferences › Navigation: Home, Fit or Off).

Fixes #689

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/lib.rs

@@ -823,6 +823,8 @@ mod tests;
 #[cfg(test)]
 mod thread_tests;
 #[cfg(test)]
+mod view_bounds_tests;
+#[cfg(test)]
 mod world_placement_cache_tests;
 
 /// Per sketch feature: its parameter-driven dimensions' values (see [`own_values`]).