feat(compare): COMPARE overlays what differs from another drawing

COMPARE picks a drawing (or takes its path) and matches its objects with
this drawing's by what they draw: type, layer and tessellated geometry,
so an unchanged copy matches whatever its handle. Objects only here are
outlined in green, objects only in the other drawing in red, over the
current drawing, and Next / Previous zoom from one difference to the
next with a count in the prompt. A moved or edited object shows as one
of each. eXit (or Esc) clears the overlay.

Refs #970

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/modules/manage/compare.rs

@@ -0,0 +1,104 @@
+// COMPARE — show what differs from another drawing and step through it.
+//
+//   COMPARE  <added> only here (green), <removed> only in the other drawing
+//   (red) [Next/Previous/eXit]:
+
+use crate::command::{CadCommand, CmdOption, CmdResult};
+use crate::scene::compare::Comparison;
+use crate::scene::WireModel;
+use glam::DVec3;
+
+pub struct CompareCommand {
+    comparison: Comparison,
+    /// The difference shown last; `None` before the first step.
+    current: Option<usize>,
+}
+
+impl CompareCommand {
+    pub fn new(comparison: Comparison) -> Self {
+        Self {
+            comparison,
+            current: None,
+        }
+    }
+
+    fn show(&mut self, forward: bool) -> CmdResult {
+        let count = self.comparison.spots.len();
+        if count == 0 {
+            return CmdResult::NeedPoint;
+        }
+        let next = match (self.current, forward) {
+            (None, true) => 0,
+            (None, false) => count - 1,
+            (Some(index), true) => (index + 1) % count,
+            (Some(index), false) => (index + count - 1) % count,
+        };
+        self.current = Some(next);
+        let (min, max) = self.comparison.spots[next];
+        // A margin around the difference so it is seen in its context.
+        let margin = ((max - min).length() * 0.5).max(1.0);
+        CmdResult::Show {
+            p1: min - DVec3::splat(margin),
+            p2: max + DVec3::splat(margin),
+        }
+    }
+}
+
+impl CadCommand for CompareCommand {
+    fn name(&self) -> &'static str {
+        "COMPARE"
+    }
+
+    fn prompt(&self) -> String {
+        let (added, removed) = (self.comparison.added, self.comparison.removed);
+        let position = match self.current {
+            Some(index) => format!(" {}/{}", index + 1, self.comparison.spots.len()),
+            None => String::new(),
+        };
+        crate::tf!(
+            "COMPARE  {added} only here (green), {removed} only in the other drawing (red){position} [Next/Previous/eXit]:"
+        )
+        .into_owned()
+    }
+
+    fn options(&self) -> Vec<CmdOption> {
+        vec![
+            CmdOption::new("Next", "N"),
+            CmdOption::new("Previous", "P"),
+            CmdOption::new("eXit", "X"),
+        ]
+    }
+
+    fn wants_text_input(&self) -> bool {
+        true
+    }
+
+    fn on_text_input(&mut self, text: &str) -> Option<CmdResult> {
+        match text.trim().trim_start_matches('_').to_ascii_uppercase().as_str() {
+            "" | "N" | "NEXT" => Some(self.show(true)),
+            "P" | "PREVIOUS" => Some(self.show(false)),
+            "X" | "EXIT" => Some(CmdResult::Cancel),
+            _ => None,
+        }
+    }
+
+    fn on_point(&mut self, _pt: DVec3) -> CmdResult {
+        CmdResult::NeedPoint
+    }
+
+    fn on_enter(&mut self) -> CmdResult {
+        self.show(true)
+    }
+
+    fn on_escape(&mut self) -> CmdResult {
+        CmdResult::Cancel
+    }
+
+    fn on_preview_wires(&mut self, _pt: DVec3) -> Vec<WireModel> {
+        // ponytail: the overlay is cloned per frame; cache it in the scene if
+        // comparisons of very large drawings stutter.
+        self.comparison.overlay.clone()
+    }
+}
+
+inventory::submit!(crate::command::CommandRegistration { names: &["COMPARE"] });

src/modules/manage/mod.rs

@@ -2,6 +2,7 @@
 
 mod about;
 mod audit;
+pub mod compare;
 mod cui_export;
 mod cui_import;
 mod find_nonpurgeable;