refactor(offset): take polyline offsetting from cadkernel

The offset core moves out: normalising the source, the cavalier_contours
call, the sharp-corner join fixup and the conversion back. What stays is
entity work — reading an LwPolyline in, writing offset LwPolylines out,
and the per-type offsets for lines, arcs, circles, ellipses and splines,
which are analytic and never needed the polyline machinery.

`BulgeArc` moves with it, since the offset preprocessing splits over-half-
turn arcs and cannot work without it. `entities::common` now re-exports it
from the kernel, so the twelve modules already reaching for
`entities::common::BulgeArc` are untouched.

`norm_rad` was a fourth copy of angle normalisation, after the three
removed from trim, fillet and explode. It now aliases the kernel's.

cavalier_contours leaves this crate's manifest: nothing here calls it any
more, and it arrives through the kernel's `offset` feature instead, which
acadifc forwards.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates/ocs_plugin_api/Cargo.toml

@@ -9,7 +9,7 @@ license = "GPL-3.0-only"
 # Pulled in only by the `host` feature, which adds the `acadrust`-typed
 # `HostApi` runtime surface. The default crate stays dependency-free so engine
 # crates and external tooling can depend on the manifest/ribbon contract cheaply.
-acadrust = { package = "acadifc", git = "https://github.com/OpenAEC-Foundation/acadifc.git", rev = "08a3d71", optional = true, features = ["serde"] }
+acadrust = { package = "acadifc", git = "https://github.com/OpenAEC-Foundation/acadifc.git", rev = "0a61cf9", optional = true, features = ["serde"] }
 
 # Runtime IPC and serialization (host feature only).
 interprocess = { version = "2", optional = true }