[P6] Mock data adapter (src/db_mock.sh)

[fissible]

Static fixture data adapter for UI development.

Functions:

• shql_db_list_tables → hardcoded list
• shql_db_describe → hardcoded DDL string
• shql_db_fetch → hardcoded TSV rows
• shql_db_query → hardcoded result set

Activated when SHQL_MOCK=1 is set. No sqlite3 dependency.

Effort: S (1–2h) | Phase: 6 (precondition for Phase 5)

[fissible]

Done — implemented in Phase 6.1. src/db_mock.sh: shql_mock_load_recent, shql_db_list_tables, shql_db_describe, shql_db_fetch, shql_db_query with fixture data.