Add postcode boundary calculation

pipeline/transform/postcode_boundaries/__init__.py

@@ -0,0 +1,23 @@
+"""Generate postcode boundary polygons from OA boundaries, INSPIRE parcels, and UPRN data.
+
+Produces per-district GeoJSON files compatible with the Rust server's postcode loader.
+Each postcode gets a polygon (or MultiPolygon) guaranteed to be contained within its
+Output Area(s), with 100% OA coverage and no overlaps between postcodes within an OA.
+
+Algorithm per OA:
+  1. Single-postcode OA → entire OA polygon assigned to that postcode
+  2. Multi-postcode OA:
+     a. Assign INSPIRE parcels to postcodes via UPRN point-in-polygon majority vote
+     b. Union INSPIRE parcels per postcode, clip to OA → "claimed" area
+     c. Distribute remaining (unclaimed) OA area via Voronoi of UPRN points
+     d. Final polygon = claimed + Voronoi share
+
+Memory-efficient design (<12GB total):
+  - INSPIRE polygons stored as raw coordinate bytes in parquet; Shapely objects built
+    lazily per-OA via numpy bbox pre-filter (~100-500 candidates at a time)
+  - UPRNs kept as sorted polars DataFrame with offset dict (Arrow storage, ~1.2GB)
+  - OA processing runs sequentially (no multiprocess INSPIRE duplication)
+
+Output format: {output}/units/{DISTRICT}.geojson with properties.postcodes and
+properties.mapit_code fields matching server-rs/src/data/postcodes.rs expectations.
+"""