[P6] SQLite adapter (src/db.sh)

[fissible]

Real sqlite3 backend wired behind the adapter interface.

Functions:

• shql_db_list_tables <db_path>
• shql_db_describe <db_path> <table>
• shql_db_fetch <db_path> <table> [limit] [offset]
• shql_db_query <db_path> <sql>

All output as TSV. Errors to stderr. No UI calls in this file.

Effort: L (~1 day) | Phase: 6

[fissible]

Done — implemented in Phase 6.3. src/db.sh: shql_db_list_tables, shql_db_describe, shql_db_fetch, shql_db_query against real sqlite3. 24 integration tests passing.