v2.2.0b1 feat: add build123d FTC conversion and validation

[Yulia2233]

Summary

Add an execution-based build123d → FTC pipeline: isolated tracing, exact profile reconstruction, native feature/boolean replay, generated Python modules and assemblies, API profiling, and independent geometry/STEP acceptance.

• Add recorded scale, affine transform, signed offset, draft, tapered/until extrusion, and multisection sweep features, plus all-output set booleans and ordered graph input references.
• Fix empty-output fallback, validation of the actual build return value and assembly placements, Compound boolean semantics, dropped cut components, per-body blend selections, split empty sides, ignored torus angles, and Face/Wire hole handling.
• Capture Builder/algebra kernel booleans and planar Shell-to-Solid construction; normalize 2D twist origins and reconstruct holed sweeps explicitly. Large polygon-hole profiles use compact per-loop sketches.
• Preserve exact rational/periodic curve data and span-aware mass/area integration. Unknown operations and unsupported geometry remain explicit failures.

Verification

• Fresh SimpleCADAPI converter/runtime regressions: 225 passed, plus 137 subtests.
• Fresh build123d tracing and unknown-API regressions: 27 passed.
• Fresh original 50-model capture and generation: 50/50 complete. Basic geometry: 50/50. Strict STEP comparison: 49/50; the tea-cup reference difference still produces invalid topology.
• Additional 41-model corpus exercised in separate kernel environments. Initial strict acceptance was 21/41; follow-up fixes independently recovered cases 63, 73, 79, 87–90, 95, 96.
• git diff --check clean.

CAD impact and known limits

Acceptance uses the actual returned Solid/Part/Assembly in world coordinates and checks declared BODIES consistency, validity, volume, surface area, solid count, absolute bounds and optional two-way STEP differences. Geometry thresholds were not relaxed.

This is sampled converter coverage. Remaining failures include the actual 625-hole model, holed-pipe/assembly consistency, selection failures, geometric mismatches, unsupported freeform constructions and a capture timeout. Non-full torus tube sections are explicitly rejected. Full topology history is partial for the new set/deformation operations; geometric inputs, output slots and graph replay are retained.

A full repository run also found three failures reproduced in an unchanged pre-review checkout (6a63494) (blend diagnostics, FreeCAD articulated-subassembly joint count, and the Gordon-surface volume assertion). These were left unchanged at the user's request.

Documentation

The converter README, API research/coverage inventory, benchmark notes, and regression fixtures are included. Benchmark notes describe the initial corpus run; the follow-up evidence above is reported separately.

Delivery scope

This PR preserves the current checkout and commit history. The new fix commit is 3c15754; it changes 23 files. No further feature edits or release-version bump were made during submission.

The target GitHub dev is 2f81fcd; this branch contributes three converter-related commits from their shared development baseline. The target merge was checked with git merge-tree --write-tree and has no conflicts. The tests above ran on feature HEAD 3c15754; CI should verify the combined development state.