fix(polyline): correct wide-polyline fill band arc center + add LWDISPLAY toggle

Wide LWPolyline/Polyline2D fill bands now sit on the centerline:
- polyline_segment_fill arc center matched explode.rs / acadrust convention
  (signed d_perp = r·(1-b²)/(1+b²), handles major arcs correctly)
- per-vertex hw0/hw1 taper applied along arc; tess density 24/circle
- synced_hatch_models subtracts hatch_offset from band boundary so it lives
  in the same offset-rel space as the wire tessellation

LWDISPLAY:
- new command `LWDISPLAY [ON|OFF]` toggles header.lineweight_display
- LWT status-bar pill (between ORTHO and POLAR) drives the same flag
- bump_geometry() on change so the viewport re-tess immediately

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/app/commands.rs

@@ -4506,6 +4506,38 @@ impl H7CAD {
                     self.command_line.push_error("Usage: LTSCALE [value]");
                 }
             }
+            cmd if cmd == "LWDISPLAY" || cmd.starts_with("LWDISPLAY ") => {
+                let val_str = cmd.trim_start_matches("LWDISPLAY").trim();
+                let parsed: Result<Option<bool>, ()> =
+                    match val_str.to_ascii_uppercase().as_str() {
+                        "" => Ok(None),
+                        "ON" | "1" | "TRUE" => Ok(Some(true)),
+                        "OFF" | "0" | "FALSE" => Ok(Some(false)),
+                        _ => Err(()),
+                    };
+                match parsed {
+                    Err(_) => self
+                        .command_line
+                        .push_error("Usage: LWDISPLAY [ON|OFF]"),
+                    Ok(Some(v)) => {
+                        self.push_undo_snapshot(i, "LWDISPLAY");
+                        self.tabs[i].scene.document.header.lineweight_display = v;
+                        self.tabs[i].scene.bump_geometry();
+                        self.tabs[i].dirty = true;
+                        self.command_line.push_output(&format!(
+                            "LWDISPLAY {}",
+                            if v { "ON" } else { "OFF" }
+                        ));
+                    }
+                    Ok(None) => {
+                        let v = self.tabs[i].scene.document.header.lineweight_display;
+                        self.command_line.push_output(&format!(
+                            "LWDISPLAY = {}",
+                            if v { "ON" } else { "OFF" }
+                        ));
+                    }
+                }
+            }
             cmd if cmd == "CELTSCALE" || cmd.starts_with("CELTSCALE ") => {
                 let val_str = cmd.trim_start_matches("CELTSCALE").trim();
                 if val_str.is_empty() {

src/app/mod.rs

@@ -401,6 +401,8 @@ pub enum Message {
     ToggleGrid,
     /// Toggle orthogonal drawing constraint — F8 / ORTHO status-bar button.
     ToggleOrtho,
+    /// Toggle LWDISPLAY header flag — LWT status-bar button.
+    ToggleLineweightDisplay,
     /// Toggle polar-angle constraint — F10 / POLAR status-bar button.
     TogglePolar,
     /// Set polar tracking angle increment (right-click POLAR button).

src/app/update.rs

@@ -2509,6 +2509,16 @@ impl H7CAD {
                 }
                 Task::none()
             }
+            Message::ToggleLineweightDisplay => {
+                let i = self.active_tab;
+                if i < self.tabs.len() {
+                    let h = &mut self.tabs[i].scene.document.header;
+                    h.lineweight_display = !h.lineweight_display;
+                    self.tabs[i].scene.bump_geometry();
+                    self.tabs[i].dirty = true;
+                }
+                Task::none()
+            }
             Message::TogglePolar => {
                 self.polar_mode ^= true;
                 if self.polar_mode {

src/app/view.rs

@@ -544,6 +544,7 @@ impl H7CAD {
                         tab.scene.annotation_scale,
                         self.scale_popup_open,
                         scale_pill_enabled,
+                        tab.scene.document.header.lineweight_display,
                     )
                 })
                 .width(Fill)