feat: annotation scale system (CANNOSCALE, PSLTSCALE, MultiLeader, Leader)

- Scale popup dropdown replaces cycle behavior; paper-space viewport scale
  editable when a viewport is active/selected
- CANNOSCALE persisted via DXF $USERR1 (read on open, write on save)
- PSLTSCALE: linetype dash lengths scaled by viewport anno_scale so dashes
  appear uniform physical size in paper space
- MultiLeader: arrow size, dogleg length, and text height scale with
  anno_scale; leader line paths (world coords) stay fixed
- Leader: arrow size and hookline length scale with anno_scale; also
  corrects world_offset subtraction that was previously deferred to the
  TruckObject::Lines path

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

src/ui/scale_popup.rs

@@ -0,0 +1,126 @@
+//! Scale picker dropdown — annotation scale (model space) or viewport scale (paper space).
+//!
+//! Rendered as a floating overlay above the status bar, same pattern as snap_popup.
+
+use iced::widget::{button, column, container, mouse_area, row, text};
+use iced::{Background, Border, Color, Element, Fill, Length, Padding, Theme};
+
+use crate::app::Message;
+
+/// (label, annotation_scale_multiplier, viewport_scale_factor)
+/// annotation_scale = multiplier for text/dim sizes (50.0 for "1:50")
+/// vp_scale = custom_scale value on the Viewport entity (0.02 for "1:50")
+const COMMON_SCALES: &[(&str, f32, f64)] = &[
+    ("1:100", 100.0, 0.01),
+    ("1:50",  50.0,  0.02),
+    ("1:20",  20.0,  0.05),
+    ("1:10",  10.0,  0.10),
+    ("1:5",   5.0,   0.20),
+    ("1:2",   2.0,   0.50),
+    ("1:1",   1.0,   1.00),
+    ("2:1",   0.5,   2.00),
+    ("5:1",   0.2,   5.00),
+    ("10:1",  0.1,   10.00),
+];
+
+#[allow(dead_code)]
+pub fn anno_scale_for_label(label: &str) -> f32 {
+    COMMON_SCALES
+        .iter()
+        .find(|&&(l, _, _)| l == label)
+        .map(|&(_, a, _)| a)
+        .unwrap_or(1.0)
+}
+
+/// Full-screen overlay: transparent click-catcher + scale list panel pinned bottom-right.
+///
+/// - `is_model`: true = model space (dispatches SetAnnotationScale), false = paper space (SetViewportScale).
+/// - `current_anno_scale`: current annotation_scale from Scene (used to highlight active row in model space).
+/// - `viewport_scale`: current vp.custom_scale (used to highlight in paper space).
+pub fn scale_popup_overlay(
+    is_model: bool,
+    current_anno_scale: f32,
+    viewport_scale: Option<f64>,
+) -> Element<'static, Message> {
+    let rows: Vec<Element<'static, Message>> = COMMON_SCALES
+        .iter()
+        .map(|&(label, anno_scale, vp_scale)| {
+            let active = if is_model {
+                (current_anno_scale - anno_scale).abs() < 0.001 * current_anno_scale.max(0.001)
+            } else {
+                viewport_scale
+                    .map(|vs| (vs - vp_scale).abs() < 0.001 * vp_scale.max(0.001))
+                    .unwrap_or(false)
+            };
+            let msg = if is_model {
+                Message::SetAnnotationScale(anno_scale)
+            } else {
+                Message::SetViewportScale(vp_scale)
+            };
+            scale_row(label, active, msg)
+        })
+        .collect();
+
+    let panel = container(column(rows))
+        .style(|_: &Theme| container::Style {
+            background: Some(Background::Color(PANEL_BG)),
+            border: Border {
+                color: PANEL_BORDER,
+                width: 1.0,
+                radius: 3.0.into(),
+            },
+            ..Default::default()
+        })
+        .width(Length::Fixed(120.0));
+
+    let positioned = container(panel)
+        .align_right(Fill)
+        .align_bottom(Fill)
+        .padding(Padding {
+            bottom: 27.0,
+            right: 4.0,
+            top: 0.0,
+            left: 0.0,
+        })
+        .width(Fill)
+        .height(Fill);
+
+    mouse_area(positioned)
+        .on_press(Message::CloseScalePopup)
+        .into()
+}
+
+fn scale_row(label: &'static str, active: bool, msg: Message) -> Element<'static, Message> {
+    let check = text(if active { "✓" } else { "  " })
+        .size(11)
+        .color(if active { CHECK_COLOR } else { Color::TRANSPARENT })
+        .width(Length::Fixed(14.0));
+
+    let lbl = text(label)
+        .size(11)
+        .color(if active { LABEL_ON } else { LABEL_OFF });
+
+    let content = row![check, lbl].spacing(6).align_y(iced::Center);
+
+    button(content)
+        .on_press(msg)
+        .style(|_: &Theme, status| button::Style {
+            background: Some(Background::Color(match status {
+                button::Status::Hovered => ROW_HOVER,
+                _ => Color::TRANSPARENT,
+            })),
+            ..Default::default()
+        })
+        .width(Fill)
+        .padding([4, 10])
+        .into()
+}
+
+// ── Colours ───────────────────────────────────────────────────────────────
+
+const PANEL_BG: Color = Color { r: 0.15, g: 0.15, b: 0.15, a: 1.0 };
+const PANEL_BORDER: Color = Color { r: 0.32, g: 0.32, b: 0.32, a: 1.0 };
+const ROW_HOVER: Color = Color { r: 0.22, g: 0.22, b: 0.22, a: 1.0 };
+const CHECK_COLOR: Color = Color { r: 0.35, g: 0.75, b: 1.00, a: 1.0 };
+const LABEL_ON: Color = Color { r: 0.92, g: 0.92, b: 0.92, a: 1.0 };
+const LABEL_OFF: Color = Color { r: 0.65, g: 0.65, b: 0.65, a: 1.0 };