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/scene/mod.rs

@@ -421,12 +421,19 @@ impl Scene {
 
         if let EntityType::Viewport(vp) = e {
             let is_active = self.active_viewport == Some(h);
-            let color = if vp.id == 1 {
+            let is_locked = vp.status.locked;
+            let color = if sel && vp.id != 1 {
+                // Selected viewport — bright white highlight.
+                [1.0, 1.0, 1.0, 1.0]
+            } else if vp.id == 1 {
                 // Overall paper-space viewport — subtle grey.
                 [0.40, 0.40, 0.40, 1.0]
             } else if is_active {
                 // Active (entered) viewport — bright yellow.
                 [1.0, 0.90, 0.20, 1.0]
+            } else if is_locked {
+                // Locked viewport — orange tint to indicate scale is frozen.
+                [0.90, 0.55, 0.10, 1.0]
             } else {
                 // Normal user viewport — cyan.
                 [0.0, 0.75, 0.75, 1.0]