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/app/mod.rs

@@ -81,6 +81,8 @@ pub(super) struct H7CAD {
     page_setup_offset_y: String,
     /// Plot rotation in degrees: "0" | "90" | "180" | "270".
     page_setup_rotation: String,
+    /// Plot scale: "Fit" | "1:1" | "1:2" | "1:4" | "1:5" | "1:10" | "1:20" | "1:50" | "1:100" | "2:1".
+    page_setup_scale: String,
 }
 
 #[derive(Debug, Clone)]
@@ -281,6 +283,7 @@ pub enum Message {
     PageSetupOffsetYEdit(String),
     /// User changed plot rotation.
     PageSetupRotation(String),
+    PageSetupScale(String),
     /// Apply the changes entered in Page Setup.
     PageSetupCommit,
     // ── Plot / Export ─────────────────────────────────────────────────────
@@ -327,6 +330,7 @@ impl H7CAD {
             page_setup_offset_x: "0.0".to_string(),
             page_setup_offset_y: "0.0".to_string(),
             page_setup_rotation: "0".to_string(),
+            page_setup_scale: "Fit".to_string(),
         };
         app.sync_ribbon_layers();
         app