Constraint glyphs: one per constrained entity, beside it, readable

Fixes #43

- Two-entity constraints (parallel, tangent, equal, ...) get a glyph on each entity.
- Glyphs sit beside their entity (offset along its screen normal), step along it when crowded,
  with a faint leader when pushed away; 15 px boxes with the toolbar's constraint icons
  (H/V stay letters), coloured from the theme's glyph tokens in both themes.
- Hovering a glyph lights both of its glyphs and its curves/points and names the constraint.
- Removes the old duplicate glyph drawer in viewport.rs (it drew a second, green set).
- Scenario expect key `glyphs` ({constraint id: count}, none overlapping) and the scenario
  constraint_glyphs_both_entities.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

crates/ui-egui/tests/scenarios/constraint_glyphs_both_entities.json

@@ -0,0 +1,14 @@
+[
+  {"call": "engine.execute", "params": {"command": "sketch.create", "params": {"plane": "XY"}}},
+  {"call": "engine.execute", "params": {"command": "sketch.rectangle.two_point", "params": {"p0": [0, 0], "p1": [40, 20]}}},
+  {"call": "engine.execute", "params": {"command": "sketch.circle.center", "params": {"center": [60, 10], "radius": 5}}},
+  {"call": "engine.execute", "params": {"command": "sketch.line", "params": {"points": [[45, -10], [75, -10]]}}},
+  {"call": "engine.execute", "params": {"command": "sketch.line", "params": {"points": [[45, 30], [75, 30]]}}},
+  {"call": "engine.execute", "params": {"command": "sketch.constraint.parallel", "params": {"a": "l5", "b": "l6"}}},
+  {"call": "engine.execute", "params": {"command": "sketch.constraint.tangent", "params": {"a": "l5", "b": "c1"}}},
+  {"call": "engine.execute", "params": {"command": "sketch.constraint.tangent", "params": {"a": "l6", "b": "c1"}}},
+  {"call": "engine.execute", "params": {"command": "sketch.constraint.equal", "params": {"a": "l5", "b": "l6"}}},
+  {"call": "ui.view", "params": {"view": "top"}},
+  {"call": "ui.view", "params": {"view": "fit"}},
+  {"expect": {"glyphs": {"k1": 1, "k5": 2, "k6": 2, "k7": 2, "k8": 2}}}
+]