Move the --attempt-instant-ddl check to run before ghost table and binlog
streaming setup. If instant DDL succeeds, the migration completes immediately
without creating ghost tables, changelog tables, or starting binlog streaming.

Add --force-instant-ddl flag that aborts the migration if ALGORITHM=INSTANT
is not supported, preventing accidental multi-hour row-copy migrations when
the intent was an instant metadata change.

Remove the now-unused AttemptInstantDDL() method from Applier since instant
DDL is handled by attemptInstantDDLEarly() in the Migrator.

Update command-line-flags.md to document the new early execution behavior of
--attempt-instant-ddl and add documentation for --force-instant-ddl.

Add localtests/force-instant-ddl with an instant-compatible ALTER to exercise
the --force-instant-ddl success path.