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>

Cargo.toml

@@ -38,7 +38,7 @@ env_logger = "0.11"
 # The CAD stack is reached through acadifc, which re-exports the codec and
 # the geometry kernel. Aliased to `acadrust` so existing `use acadrust::…`
 # paths keep resolving.
-acadrust = { package = "acadifc", git = "https://github.com/OpenAEC-Foundation/acadifc.git", rev = "08a3d71", features = ["serde"] }
+acadrust = { package = "acadifc", git = "https://github.com/OpenAEC-Foundation/acadifc.git", rev = "0a61cf9", features = ["serde", "offset"] }
 dwg-thumbnailer = { path = "crates/dwg-thumbnailer" }
 flate2 = "1"
 image = { version = "0.25", default-features = false, features = ["png", "jpeg", "bmp", "tiff"] }
@@ -57,7 +57,6 @@ fontdb = "0.23"
 ttf-parser = "0.25"
 cosmic-text = "0.15"
 lyon_tessellation = "1.0.20"
-cavalier_contours = "=0.7.0"
 
 [dev-dependencies]
 naga = { version = "27", features = ["wgsl-in"] }