feat: smooth native junction boundaries

.trellis/tasks/08-17-native-rounded-junctions/implement.md

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+# Implementation
+
+1. Build a rounded junction boundary from ordered approach-edge records with
+   tangent support-line intersections and deterministic curve samples.
+2. Expose boundary mode/provenance and preserve containment fallback.
+3. Extend focused native-road tests for curved ordinary intersections.
+4. Validate Nantaizi compile/check, workbench tests, and native 3D build.