Feat: implement WCS↔UCS coordinate transform pipeline

- Add `active_ucs: Option<Ucs>` per-tab state to DocumentTab
- Add `ucs_to_wcs`, `wcs_to_ucs`, `ucs_z_axis`, `ucs_rotated_z` helpers
- Add `Camera::pick_on_plane` for ray–plane intersection against any plane
- Mouse picks project onto the active UCS XY plane instead of world XY
- Typed coordinates are converted from UCS space to WCS before dispatch
- UCS command now really activates/deactivates a UCS:
  - `UCS W` resets to WCS (clears active_ucs)
  - `UCS <name>` activates a named UCS
  - `UCS SAVE <name>` saves the current active UCS
  - `UCS ORIGIN x,y,z` shifts origin (in current UCS space)
  - `UCS X/Y/Z <angle>` rotates the UCS around its own axes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/document.rs

@@ -5,6 +5,7 @@ use crate::snap::SnapResult;
 use crate::scene::grip::GripEdit;
 use crate::scene::GripDef;
 use acadrust::{CadDocument, Handle};
+use acadrust::tables::Ucs;
 use crate::linetypes;
 use std::path::PathBuf;
 
@@ -28,6 +29,8 @@ pub(super) struct DocumentTab {
     pub(super) last_cursor_world: glam::Vec3,
     pub(super) history: HistoryState,
     pub(super) active_layer: String,
+    /// Currently active UCS. `None` means WCS (identity transform).
+    pub(super) active_ucs: Option<Ucs>,
 }
 
 impl DocumentTab {
@@ -52,6 +55,7 @@ impl DocumentTab {
             last_cursor_world: glam::Vec3::ZERO,
             history: HistoryState::default(),
             active_layer: "0".to_string(),
+            active_ucs: None,
         }
     }