Mesh: a face edge split that encroaches a feature edge splits the feature edge

Delaunay refinement split a boundary face edge at its midpoint even when that
midpoint lay inside the diametral sphere of a feature edge beside it. Each such
split halves the face vertices' distance to the curve, so they piled up against
it (the heated bracket grew face vertices 0.016 mm from a 4 mm-sized edge), the
improvement schedule could not lift the tetrahedra there to its target, and
inserted vertex after vertex around them: 29257 elements and about two minutes
to open instead of 23641 and 6 s. The SolidCraft pin move (4c91f9b) added one
interior vertex to the bracket's end faces, which changed the CDT enough to
expose it.

Following Shewchuk (1998), an encroaching midpoint now splits the encroached
feature edge instead.

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

docs/examples.md

@@ -76,8 +76,9 @@ The corpus filleted bracket (`tests/corpus/filleted-bracket`: an 8 mm L-bracket,
 hole through each leg) in steel, both hole bores clamped. The end of the upright is held at
 120 °C by a heater; the rest of its outside loses heat to still air at 20 °C
 (h = 10 W/(m² K)); steel conducts with k = 50 W/(m K), expands with α = 12 × 10⁻⁶/K and is
-stress free at 20 °C. One coupled temperature–displacement step (steady) on 23 641 tet10
-elements: about 8 s to open (meshing) and 13 s to solve.
+stress free at 20 °C. One coupled temperature–displacement step (steady) on about 23 000 tet10
+elements (the count differs a little between platforms): about 5 s to open (meshing) and 13 s to
+solve.
 
 **Look at:** Contours ▸ Temperature: the upright stays within a few degrees of the heater and
 the foot falls to 95 °C, a steady balance between conduction and the air. Contours ▸ Von Mises