New concrete base methods (backend-agnostic, use entity_get + pure math):
- AutoCADBackend.point_intersection: LINE-LINE, LINE-CIRCLE, CIRCLE-CIRCLE
with ref_x/ref_y disambiguation for multiple candidates.
- AutoCADBackend.point_tangent: external-point-to-circle tangent touch-point
with ref_x/ref_y for side selection; correct perpendicularity formula
(T-F)⊥(T-C) via cos(θ-α)=-r/d.
Registered as MCP tools: point_intersection, point_tangent.
13 new tests in tests/test_meta_tools_geometry.py.
226 tests total.
