Reference case for a fillet with a B-spline radius (#810)

Adds oracle case 81 (a box edge rounded at a clamped cubic B-spline radius) with analytic expected values, and passes spline fillets through recipes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/oracle-analytic/README.md

@@ -11,7 +11,7 @@ plain Python 3 (standard library only) with one function per case. Running a scr
 python3 tools/oracle-analytic/cases_basic.py        # 01-29: boxes, holes, revolves, sketches
 python3 tools/oracle-analytic/cases_features.py     # 32-66: fillets, shells, sweeps, patterns
 python3 tools/oracle-analytic/cases_sheet_metal.py  # 67-76: sheet metal
-python3 tools/oracle-analytic/cases_fillets.py      # 77-80: variable, chord-length and tapered fillets
+python3 tools/oracle-analytic/cases_fillets.py      # 77-81: variable, chord-length, tapered and spline fillets
 ```
 
 Where a value depends on a SolidCraft convention rather than on geometry alone (a shell's sharp