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/modules/home/modify/join.rs

@@ -7,7 +7,6 @@
 //
 // Workflow: select objects then press Enter to join.
 
-use acadrust::entities::{Arc as ArcEnt, Line as LineEnt};
 use acadrust::types::Vector3;
 use acadrust::{EntityType, Handle};
 use glam::Vec3;
@@ -162,7 +161,7 @@ fn try_join_arcs(arcs: &[&(Handle, &EntityType)]) -> Option<(Vec<Handle>, Vec<En
     intervals.sort_by(|a, b| a[0].partial_cmp(&b[0]).unwrap());
 
     // Try to merge into one contiguous arc
-    let mut merged_start = intervals[0][0];
+    let merged_start = intervals[0][0];
     let mut merged_end   = intervals[0][1];
     for &[s, e] in &intervals[1..] {
         let span = ((e - merged_end) + 360.0) % 360.0;