v1.8.1 — fix: single-start threadedShaft always smooth; deprecate .boolean (#254)

Fix. threadedShaft(build: .boolean) produced a faceted, disconnected thread — a helical scatter of rectangular notches instead of a continuous groove — because it forced the screw-loft boolean cut path, whose tightly-wound helical cutter is the classic OCCT BOP failure (cf. #213/#225). The solid was isValid with ~the right volume, so only rendering exposed it.

.boolean only existed to clamp a supposed crest "overshoot" (#222) — but #232 established that overshoot is a Bnd_Box control-hull artifact. Verified here: the direct build's crest measures exactly nominal by both boundingBoxOptimal() and mesh vertices, while .bounds over-reads +14–21%.

With no remaining reason to prefer it, single-start coaxial-cylinder threads now take the smooth, boolean-free direct build (#213) for every build mode, and ThreadBuild.boolean is deprecated (treated as .auto). Use .auto or .direct.

.auto/.direct single-start behaviour is unchanged. Swift-only — no xcframework rebuild.

Known limitation: multi-start (starts > 1) and non-cylinder targets still use the faceted cut path (no direct alternative yet) — tracked separately.

Full thread suite (49 tests) green. Closes #254.

Note: the separately-reported helical-flank "ridges" (#255) are a renderer normals artifact, not a geometry bug — see OCCTSwiftViewport#81.