feat: canonical SQL DDL + schema validator + migration tool
- schema/canonical.sql: 29 tables across 3 databases, CHECK constraints,
foreign keys, 13 indexes, WAL mode, schema_version tracking
- tools/validate-schema.ts: applies DDL to in-memory SQLite, extracts
PRAGMA table_info + sqlite_master metadata as JSON
- tools/migrate-db.ts: CLI for Tauri→Electrobun data migration with
atomic transaction, version fencing, INSERT OR IGNORE
- docs/SWITCHING.md: migration guide with prerequisites and troubleshooting
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