docs(database-legacy): rewrite stale README to match the real DBAL API

The README described a PDO implementation (PdoDatabase, PdoSelect,
PdoDatabase::createSqlite, getPdo()) that no longer exists — the src is now 14
Doctrine DBAL classes (DBALDatabase, DBAL{Select,Insert,Update,Delete},
DBALSchema, DBALTransaction, + interfaces), namespace Waaseyaa\Database.

Rewrote it to document the actual API, call out the deliberate directory-name vs
namespace split (per ADR 007), and record that "legacy" does NOT mean removable:
this is the sole live DB layer (16 dependents) and the attempted retirement was
reverted 2026-05-26.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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