Add silo_metadata, Arrow extension type, and CRS-aware DuckToArrow

- silo_metadata(blob) returns header without decoding: geometry_type,
  vertex_type, scale as a STRUCT
- Register "queryfarm.geosilo" Arrow extension type for transparent
  encode/decode over Arrow IPC transports
- DuckToArrow callback detects CRS on GEOMETRY type to pick scale
  (was hardcoded to 1e7)
- Remove OpenSSL dependency from vcpkg.json
- Add silo_metadata tests (35 assertions total)
- Document Arrow extension type and silo_metadata in README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rustyconover