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>

src/entities/common.rs

@@ -582,90 +582,12 @@ pub fn parse_angle_deg(value: &str) -> Option<f64> {
     Some(if neg { -total } else { total })
 }
 
-/// Bulge → arc geometry for a polyline segment.
+/// Bulge → arc geometry for a polyline segment, from the kernel.
 ///
-/// DXF/DWG polyline arcs are encoded as a bulge factor on each vertex —
-/// `bulge = tan(theta/4)` where `theta` is the included angle of the arc
-/// from `p0` to `p1`. Sign convention: positive bulge = CCW from p0 to p1,
-/// negative = CW. `|bulge| = 1` is a half-circle.
-///
-/// This struct centralises the (formerly duplicated) math that takes
-/// `(p0, p1, bulge)` and produces the canonical `(center, radius,
-/// start_angle, sweep)` quadruple. Callsites pick the fields they need.
-#[derive(Clone, Copy, Debug)]
-pub struct BulgeArc {
-    pub center: [f64; 2],
-    pub radius: f64,
-    /// Angle from center to p0 (atan2, range -π..π).
-    pub start_angle: f64,
-    /// Angle from center to p1 (atan2, range -π..π).
-    pub end_angle: f64,
-    /// Signed sweep from p0 to p1. Positive ⇒ CCW (bulge > 0),
-    /// negative ⇒ CW (bulge < 0). For exact half-turns the sign of
-    /// `bulge` decides the direction.
-    pub sweep: f64,
-}
-
-impl BulgeArc {
-    /// Build from endpoints + bulge. Returns `None` for degenerate input
-    /// (chord ≈ 0 or |bulge| ≈ 0).
-    pub fn from_bulge(p0: [f64; 2], p1: [f64; 2], bulge: f64) -> Option<Self> {
-        let chord_x = p1[0] - p0[0];
-        let chord_y = p1[1] - p0[1];
-        let chord_len = (chord_x * chord_x + chord_y * chord_y).sqrt();
-        if chord_len < 1e-12 || bulge.abs() < 1e-12 {
-            return None;
-        }
-        let b = bulge;
-        let b2 = b * b;
-        // r = chord · (1 + b²) / (4·|b|)
-        let r = chord_len * (1.0 + b2) / (4.0 * b.abs());
-        // d_perp = signed distance from chord midpoint to arc center
-        //        = r · (1 - b²) / (1 + b²) = r · cos(theta/2)
-        let d_perp = r * (1.0 - b2) / (1.0 + b2);
-        let mx = (p0[0] + p1[0]) * 0.5;
-        let my = (p0[1] + p1[1]) * 0.5;
-        // Left perpendicular to chord (90° CCW).
-        let perp_x = -chord_y / chord_len;
-        let perp_y = chord_x / chord_len;
-        let sign = b.signum();
-        let cx = mx + sign * d_perp * perp_x;
-        let cy = my + sign * d_perp * perp_y;
-        let a0 = (p0[1] - cy).atan2(p0[0] - cx);
-        let a1 = (p1[1] - cy).atan2(p1[0] - cx);
-        // Wrap sweep to match bulge sign: bulge>0 ⇒ positive (CCW),
-        // bulge<0 ⇒ negative (CW). Falls back to ±τ for half-turns.
-        const TAU: f64 = std::f64::consts::TAU;
-        let mut sweep = a1 - a0;
-        if bulge > 0.0 {
-            if sweep <= 0.0 {
-                sweep += TAU;
-            }
-        } else if sweep >= 0.0 {
-            sweep -= TAU;
-        }
-        if sweep.abs() < 1e-9 {
-            sweep = if bulge > 0.0 { TAU } else { -TAU };
-        }
-        Some(Self {
-            center: [cx, cy],
-            radius: r,
-            start_angle: a0,
-            end_angle: a1,
-            sweep,
-        })
-    }
-
-    /// Sample a point on the arc at parameter `t ∈ [0, 1]`. `t=0` ↦ p0,
-    /// `t=1` ↦ p1, walks along the signed sweep direction.
-    pub fn sample(&self, t: f64) -> [f64; 2] {
-        let a = self.start_angle + self.sweep * t;
-        [
-            self.center[0] + self.radius * a.cos(),
-            self.center[1] + self.radius * a.sin(),
-        ]
-    }
-}
+/// Re-exported rather than imported at each call site so the twelve modules
+/// that already reach for `entities::common::BulgeArc` keep working, and so
+/// there is one obvious place to see that the maths moved out.
+pub use acadrust::kernel::geom2d::BulgeArc;
 
 /// Triangulate the solid bands a `wide_fills` returns into the flat WCS f64
 /// triangle list `TruckEntity::pick_tris` carries, so a wide polyline is