test: reference cases for variable, chord-length and tapered fillets (#776)

Four analytic reference cases for variable, to-zero, chord-length and taper-to-fit fillets, with the formulas in tools/oracle-analytic.

tools/oracle-analytic/README.md

@@ -2,15 +2,16 @@
 
 Closed-form expected values for the cases in [`tests/oracle/`](../../tests/oracle/) whose shape
 has one: prisms, cylinders, cones and frustums, spheres and caps, tori and solids of revolution
-(Pappus), constant-radius fillets on straight edges, shells, patterns of these, and folded sheet
-metal (flats plus bends as annular sectors). Each script is plain Python 3 (standard library only)
-with one function per case. Running a script from the repository root rewrites the
+(Pappus), fillets on straight edges (a constant radius, or one varying along the edge), shells,
+patterns of these, and folded sheet metal (flats plus bends as annular sectors). Each script is
+plain Python 3 (standard library only) with one function per case. Running a script from the repository root rewrites the
 `expected.json` of its cases with `"source": "analytic"` and a `method` string giving the formulas:
 
 ```sh
 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
 ```
 
 Where a value depends on a SolidCraft convention rather than on geometry alone (a shell's sharp