v1.4.0: schema diff since last migration

Added

• Schema diff: see what changed since your last migration. After each migration Truss records the previous schema as a baseline and compares it against the current one. The dashboard gains a "Changes" panel that tints added and changed tables and lists every added, removed, or changed table, column, index, and foreign key, and a new php artisan truss:diff command prints the same diff in the terminal (handy in CI). Structure only, never row data. The baseline is a structure-only JSON file, the only thing Truss writes to disk, stored at truss/baselines/{connection}.json on the disk set by truss.diff.disk. Set TRUSS_DIFF_ENABLED=false to turn the feature off entirely so nothing is written to disk.