fix(feedback): resolve VDE-077 — rename Continuity::C0 to G0, verify PCurve is NurbsCurve alias

- Issue 1 (n_sided_fill PCurve): Verified not reproducible. PCurve is a public
  using-alias (PCurve = curves::NurbsCurve) in vde/brep/trimmed_surface.h.
  Header and implementation both use NurbsCurve consistently.
- Issue 2 (Continuity enums): Renamed Continuity::C0 → G0 to match
  ContinuityLevel::G0 naming. C0 was unused in entire codebase.
  Full unification deferred (architecture-level — ContinuityLevel needs None
  for analysis results while Continuity serves active operations).

include/vde/curves/continuity_types.h

@@ -6,7 +6,7 @@
  * 所有 vde::curves 模块共享的连续性相关枚举类型集中定义于此。
  * 避免在多个公共头文件中重复定义相同枚举导致命名冲突。
  *
- * - Continuity: 几何连续性等级（G1/G2/G3，可选 C0 位置连续）
+ * - Continuity: 几何连续性等级（G1/G2/G3，可选 G0 位置连续）
  * - ContinuityLevel: 连续性等级（含 None/G0/G1/G2/G3）
  *
  * @ingroup curves