feat: Dynamic Input overlay (DYNMODE / F12)

- overlay.rs: dynamic_input_overlay() canvas widget draws a semi-transparent
  coordinate tooltip near the cursor; shows X/Y in absolute mode or
  dist/angle relative to last_point when a command base point exists
- statusbar: DYN toggle pill with F12 tooltip (calls ToggleDynInput)
- app/mod.rs: dyn_input: bool field (default true), ToggleDynInput message
- view.rs: builds DYN label and pushes overlay into viewport_stack when
  dyn_input=true and a command is active
- document.rs: last_cursor_screen: iced::Point — updated on every ViewportMove
- ROADMAP.md: Dynamic Input overlay marked ✅

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

src/app/document.rs

@@ -8,6 +8,7 @@ use acadrust::{CadDocument, Handle};
 use acadrust::tables::Ucs;
 use crate::linetypes;
 use std::path::PathBuf;
+use iced;
 
 // ── Per-document tab state ─────────────────────────────────────────────────
 
@@ -27,6 +28,7 @@ pub(super) struct DocumentTab {
     pub(super) wireframe: bool,
     pub(super) visual_style: String,
     pub(super) last_cursor_world: glam::Vec3,
+    pub(super) last_cursor_screen: iced::Point,
     pub(super) history: HistoryState,
     pub(super) active_layer: String,
     /// Currently active UCS. `None` means WCS (identity transform).
@@ -57,6 +59,7 @@ impl DocumentTab {
             wireframe: false,
             visual_style: "Shaded".into(),
             last_cursor_world: glam::Vec3::ZERO,
+            last_cursor_screen: iced::Point::ORIGIN,
             history: HistoryState::default(),
             active_layer: "0".to_string(),
             active_ucs: None,