Add --extension flag to load SQLite extensions

[vmvarela]

Description

Allow loading SQLite extensions to extend SQL functionality (e.g., spellfix, percentile, custom functions).

Examples

sql-pipe --extension /path/to/spellfix.so data.csv \
  'SELECT * FROM t WHERE name MATCH "alice"'

Acceptance Criteria

• --extension <path> flag loads SQLite extension
• Can be specified multiple times for multiple extensions
• Error message if extension can't be loaded
• Extensions are available for the query
• All existing tests pass
• New tests cover extension loading (if test extensions are available)

Notes

• Implementation: ~30 lines
• Use sqlite3_load_extension() C API
• Power-user escape hatch that adds zero complexity to common case
• Low priority but unlocks niche use cases without bloating binary