fix(snap): correct snap symbols + drop bogus midpoints on circles (#34)

Two bugs from #34:

1. Hovering a circle showed the Center snap as a diamond instead of
   the conventional circle outline.
2. Hovering a circle also showed Midpoint hits (triangle marker)
   even though a closed curve has no midpoint.

(2) came from a "tessellated curve" fallback in the Midpoint snap
path that produced a midpoint for every chord of the polyline
approximation. Drop that fallback — only entities with explicit
`key_vertices` (Line, LwPolyline) contribute per-segment midpoints
now. Curves that DO have a single well-defined midpoint (arcs, and
later splines) declare it explicitly via a new
`SnapHint::Midpoint`; `arc.rs` emits one at the arc-length centre.

(1) — and the broader "everything past Endpoint / Midpoint / Grid
was a diamond" inconsistency — fixed by giving every `SnapType` a
distinct marker in `SelectionCanvas::draw`:

- Center: hollow circle
- Node: circle with inscribed X
- Quadrant: diamond (unchanged)
- Intersection: X
- Apparent Intersection: X inside a square
- Insertion: two overlapping rectangles (tag glyph)
- Perpendicular: right-angle hook
- Tangent: circle with a bar across the top
- Nearest: hourglass / bowtie
- Extension: three dots
- Parallel: two parallel diagonal bars
- Endpoint / Midpoint / Grid: unchanged

Closes #34

src/snap/mod.rs

@@ -334,6 +334,7 @@ impl Snapper {
                     SnapHint::Node => SnapType::Node,
                     SnapHint::Quadrant => SnapType::Quadrant,
                     SnapHint::Insertion => SnapType::Insertion,
+                    SnapHint::Midpoint => SnapType::Midpoint,
                 };
                 if self.is_on(snap_type) {
                     try_pt(world, snap_type);
@@ -367,13 +368,17 @@ impl Snapper {
         }
 
         // ── Midpoint ───────────────────────────────────────────────────────
+        // Only explicit vertex sets (Line, LwPolyline) contribute per-segment
+        // midpoints. Tessellated curves (Circle, Arc, Ellipse, Spline) emit a
+        // single `SnapHint::Midpoint` snap_pt where one exists — iterating
+        // every chord here would otherwise turn a circle's tessellation into
+        // a haze of false midpoint hits. See #34.
         if self.is_on(SnapType::Midpoint) {
             for wire in wires {
                 if !wire_in_range(wire) {
                     continue;
                 }
                 if !wire.key_vertices.is_empty() {
-                    // Use explicit vertices for accurate per-segment midpoints.
                     for seg in wire.key_vertices.windows(2) {
                         let a = Vec3::from(seg[0]);
                         let b = Vec3::from(seg[1]);
@@ -381,16 +386,6 @@ impl Snapper {
                             try_pt((a + b) * 0.5, SnapType::Midpoint);
                         }
                     }
-                } else {
-                    // Tessellated curves: midpoint of whole arc (first↔last midpoint skipped
-                    // for closed curves; arcs use their own quadrant snaps instead).
-                    for seg in wire.points.windows(2) {
-                        let a = Vec3::from(seg[0]);
-                        let b = Vec3::from(seg[1]);
-                        if a.distance_squared(b) > 1e-12 {
-                            try_pt((a + b) * 0.5, SnapType::Midpoint);
-                        }
-                    }
                 }
             }
         }