mesh-compile: accept CurveRelation boundaries (Composite, Trimmed) for curve-bounded planes

[GeneralPawz]

Reported by a consumer (axioval). Affects 388 space-boundary outcomes in 3 models.

A curve-bounded plane in `ReferenceMeshCompiler` (`bounded.rs`, still refused in 0.3.12) accepts `Curve2`/`Curve3` boundaries but refuses `CurveRelation` boundaries.

Proposal

Resolve `CurveRelation::Composite` (polyline, line and arc segments, honouring each segment's sense) and `CurveRelation::Trimmed` to points before flattening, exactly as atomic `Curve2`/`Curve3` boundaries are flattened.

• Reuse the directrix reading in `directrix.rs`/`pieces.rs`, which already resolves composites and trims, including `ArcLength` from model/curve: arc-length trim selector and arc-length plan chain (for exact CUBIC transitions) #239.
• Keep the deviation report: chord bounds per segment as in mesh paths still without a proven chord bound (follow-up to #231) #232/model/curve: arc-length trim selector and arc-length plan chain (for exact CUBIC transitions) #239.

Done when

A one-segment composite wrapping a closed polyline with SameSense false compiles to the same surface as the polyline alone. Composites mixing lines and arcs, and trimmed basis curves, compile with certified bounds. Unsupported relation kinds are refused by name.

[GeneralPawz]

Released in axiolid-mesh-compile 0.3.13. Composite and trimmed relation boundaries now work for curve-bounded planes, honouring segment sense and every trim selector, ArcLength included. Rounding-level gaps are welded, so curved boundaries meet their budget.