- Install PostGIS (postgresql--postgis-3) in the final image.
- Add postgis and postgis_topology to init-extensions.sh.
- Add test script for PostGIS to the CI workflow.
- Update README.md to reflect PostGIS integration.
- Remove PG18 from CI matrix because of ABI incompatibility between TimescaleDB headers and the vchord-postgres PG18 runtime (missing palloc0_mul). PG18 support will be restored when VectorChord releases a stable PG18 image.