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/scene/wire_model.rs

@@ -10,6 +10,10 @@ pub enum SnapHint {
     Quadrant,
     /// Insertion point of text or block.
     Insertion,
+    /// Midpoint of a curve that has one well-defined midpoint (an arc's
+    /// arc-length centre, a spline's `t = 0.5`). Lines / polylines do
+    /// not use this — their midpoints are derived from `key_vertices`.
+    Midpoint,
 }
 
 /// Geometric primitive used by the tangent-snap engine.