Add patch polygon dataset importer

docs/registry/elements/port_shoreline.md

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+---
+id: port_shoreline
+type: element
+name: Port shoreline
+task_types: [polygon_extraction, semantic_segmentation]
+preferred_heads: [validity, context, polygon, semantic_segmentation]
+label_formats: [mask, polygon, geojson, shp]
+positive_label: port_shoreline
+negative_policy: unlabeled_is_ignore
+typical_sensors: [PMS]
+typical_resolutions_m: [1, 2]
+---
+
+# Port Shoreline
+
+Port shoreline is a narrow coastal boundary element around docks, breakwaters,
+quays, and engineered harbor edges. It should be represented by mask or polygon
+annotations rather than box labels.
+
+## Required Context
+
+- Ships, piers, sea walls, roads, and bright surf are hard negatives or adjacent
+  context.
+- Preserve patch size and sensor metadata because the target is often thin.