Feat: ROADMAP Phase 1-5 batch — trim/extend, plot scale, viewport UX, mline caps, table alignment

- TRIM/EXTEND: Ray and XLine entities supported as cutting boundaries and trim targets;
  trim results in Ray→Line/Ray, XLine→Ray/Ray or Line; hover preview added
- Plot scale override: Fit/1:1/1:2/1:5/1:10/1:20/1:50/1:100/2:1 buttons added to Page Setup
  panel; PageSetupCommit applies scale to PlotSettings
- Viewport UX: locked viewport gets orange border; selected viewport gets white border
- Named view → viewport assignment: Choice dropdown in properties panel copies
  view_target/direction/height to the viewport entity
- MLine caps: perpendicular start/end cap lines added for open (non-closed) MLines
- Wipeout polygon grips: boundary vertices become editable grips when clipping_enabled+Polygonal;
  world→pixel back-projection on drag
- Table cell alignment: CellStyle.alignment (1-9) drives left/center/right + top/middle/bottom
  text positioning; cxf::measure_text() helper added
- New commands/features: DIMCONTINUE (DCO), DIMBASELINE (DBA), TOLERANCE (TOL), TABLE,
  ATTDEF, ATTEDIT, ATTDISP, BREAKATPOINT (BAP), PLOTWINDOW (PW), MLINE (ML);
  DIMSTYLE/DIMCURRENT/CLAYER/LTSCALE/CELTSCALE/SCALETEXT/XDATA/UCSICON/REGEN/DRAWORDER extended
- PLINE arc mode: A/L/CLOSE text input, bulge computation, arc preview
- Warning cleanup: removed unused imports and unreachable patterns

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

src/entities/mline.rs

@@ -38,8 +38,9 @@ impl TruckConvertible for MLine {
         // Parallel offset lines — one at +scale/2 and one at -scale/2
         // along each vertex's miter direction.
         if scale.abs() > 1e-6 {
-            for sign in [-0.5_f32, 0.5_f32] {
-                let offset = scale * sign;
+            let half = scale * 0.5;
+            for sign in [-1.0_f32, 1.0_f32] {
+                let offset = half * sign;
                 pts.push([f32::NAN; 3]);
                 for v in &self.vertices {
                     let mx = v.miter.x as f32;
@@ -63,6 +64,26 @@ impl TruckConvertible for MLine {
                     ]);
                 }
             }
+
+            // Start and end caps: perpendicular line connecting the two offset lines
+            // at the first and last vertex of an open MLine.
+            if !closed {
+                let cap_v = |v: &acadrust::entities::MLineVertex| {
+                    let mx = v.miter.x as f32;
+                    let my = v.miter.y as f32;
+                    let mz = v.miter.z as f32;
+                    let px = v.position.x as f32;
+                    let py = v.position.y as f32;
+                    let pz = v.position.z as f32;
+                    [
+                        [f32::NAN; 3],
+                        [px + mx * (-half), py + my * (-half), pz + mz * (-half)],
+                        [px + mx * half,    py + my * half,    pz + mz * half   ],
+                    ]
+                };
+                pts.extend_from_slice(&cap_v(&self.vertices[0]));
+                pts.extend_from_slice(&cap_v(&self.vertices[n - 1]));
+            }
         }
 
         let key_verts: Vec<[f32; 3]> = self