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.

docs/oracle.md

@@ -4,12 +4,12 @@ Generated by `cargo xtask oracle`. Each case in [`tests/oracle/`](../tests/oracl
 
 Where the expected values come from (the **Source** column):
 
-- **analytic**: closed-form geometry worked out by hand (prisms, cylinders, cones, spheres, tori and Pappus' theorems, constant-radius fillets on straight edges, sheet-metal bends as annular sectors). The scripts in [`tools/oracle-analytic/`](../tools/oracle-analytic/) compute them and give the formulas.
+- **analytic**: closed-form geometry worked out by hand (prisms, cylinders, cones, spheres, tori and Pappus' theorems, fillets on straight edges with a constant radius or one varying along the edge, sheet-metal bends as annular sectors). The scripts in [`tools/oracle-analytic/`](../tools/oracle-analytic/) compute them and give the formulas.
 - **freecad <version>**: the same recipe built with FreeCAD's public Python scripting API by [`tools/freecad-oracle/`](../tools/freecad-oracle/), an independent kernel run as an external program. FreeCAD is not a dependency of SolidCraft. Where the two kernels legitimately build a different shape (for example the corner patch where fillets meet), only volume and area are compared, within the tolerance the case states.
 
 No expected value comes from a commercial CAD product.
 
-**76 / 76 cases pass**; sources: 55 analytic, 21 freecad. 0 known failure(s) tracked in issues; 0 case(s) without expected values yet.
+**80 / 80 cases pass**; sources: 59 analytic, 21 freecad. 0 known failure(s) tracked in issues; 0 case(s) without expected values yet.
 
 | Case | Source | Result | Surface distance mm (max / mean) | Method | Notes |
 |---|---|---|---|---|---|
@@ -89,6 +89,10 @@ No expected value comes from a commercial CAD product.
 | 74-sm-unfold-refold | analytic | pass |  | U channel 80x50 (t 2.5, R 2.5, base y 5..45, flanges z 5..20) with the flat-pattern slot x 35..45, y -12..10 refolded: BA = (pi/2)(R + K t) = 5.654866776, so the slot removes base y 5..10, the whole y=0 bend and the flange from z 5 to 5 + (12 - (BA - 5)) = 16.345133224; x-segments 0..35 / 35..45 / 45..80 | unfold/refold maps flat distance through the K-factor neutral layer (K 0.44) |
 | 75-sm-bracket-holes | analytic | pass |  | base x 0..75, y 5..40, z 0..2.5 (trimmed 5 by each flange's setback); bend 1 (R 2.5..5 about y 5, z 5) x 0..75; flange 1 y 0..2.5, z 5..30, x 0..80; bend 2 (R 2.5..5 about x 75, z 5) y 5..40; flange 2 x 77.5..80, z 5..15, y 5..40; minus two d6 through holes at (25,20), (55,20) in the base. A = sum of prism areas - 2 x 4 joint areas (75t, 75t, 35t, 35t) + hole walls - hole discs | SolidCraft convention: the second flange trims the base under the first bend, so bend 1 stops at x 75 while flange 1's panel runs to x 80 (open corner relief, no fill). |
 | 76-sm-thickness-variant | analytic | pass |  | final rule thickness 3 (bend radius = Thickness = 3): base y 6..60 x 3; bend annulus R 3..6 about (y 6, z 6); flange y 0..3, z 6..20; extrusion along x 100 | the rule's thickness edits regenerate the whole sheet body; only the final 3 mm state matters |
+| 77-fillet-variable-linear | analytic | pass |  | box 40x30x20 with its top front edge (y=0, z=20) rounded at a radius running straight from 2 (x=0) to 5 (x=40): sections are quarter rounds, V = abc - (1 - pi/4) L (a^2 + ab + b^2) / 3; moments and the round's area integrated along the edge (tools/oracle-analytic/cases_fillets.py) |  |
+| 78-fillet-variable-to-zero | analytic | pass |  | as 77 with the radius running from 4 (x=0) down to 0 (x=40), where the round closes to the corner: V = abc - (1 - pi/4) L a^2 / 3; the round is three-sided and the corner at x=40 stays |  |
+| 79-fillet-chord-length | analytic | pass |  | box 40x30x20, top front edge rounded 4 wide across (chord c): r = c / (2 cos 45 deg) = 2 sqrt 2; V = abc - (1 - pi/4) r^2 L, A = 2(ab + bc + ca) - 2 r L + (pi/2) r L - 2 (1 - pi/4) r^2 |  |
+| 80-fillet-taper-to-fit | analytic | pass |  | plate (top 20 -> 2 wide over x 0..40, 15 high) with its top front edge filleted at 3, fit: taper. The radius is the taper law: a clamped cubic B-spline in u = x/40 on 8 equal spans, control radii min(3, 0.9 room) at the Greville abscissae, room = min(20 - 18u, 15) tan 45 deg (top face width, front face height): 3.0000, 3.0000, 3.0000, 3.0000, 3.0000, 3.0000, 3.0000, 3.0000, 3.0000, 2.4750, 1.8000. Sections are quarter rounds; V, moments and areas integrated along the edge |  |
 
 ## Not covered by a reference case