engine: draw geometry.check findings as viewport markers

geometry.check keeps its findings on the session (not an edit: not journalled,
saved or undone) while the checked design stays open; the picture and the app's
viewport draw the faces of small, sliver and near faces in the marker colour,
short edges as thick lines and a dot at each finding over everything. `mark:
false` or a clean check clears them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/lib.rs

@@ -295,6 +295,9 @@ pub struct Session {
     /// sets it on its own model (and the control channel and the command search while they
     /// run); `None` (scratch sessions, the CLI, tests) counts nothing.
     pub origin: Option<usage::Origin>,
+    /// What the last `geometry.check` found, drawn as markers while the design it checked is
+    /// open ([`view::geometry_marks`]). Not an edit: neither journalled, saved nor undone.
+    pub geometry_marks: Option<Arc<view::GeometryMarks>>,
 }
 
 impl Session {