Add ID field to Migration (parsed from INSERT INTO _migrations in Collect),
AppliedMigration type with ID+Name, and update Migrator.Applied() to return
[]AppliedMigration across all 5 backends. Matching in Up/Down/GetStatus now
checks both name and ID, so renamed migration files are still recognized as
applied if the 8-char hex ID matches.
- Change Down(n<=0) to mean "rollback all" (matching Up semantics).
CLIs now explicitly pass 1 when no count is given.
- Add 22 tests for Collect, NamesOnly, Up, Down, and GetStatus
using mock Migrator and fstest.MapFS.
- Fix shmigrate.Applied to stream from file instead of ReadFile
double-wrap; add optional FS field for embed.FS support.
- Update package doc to list all 5 backends.
- Add minimal README linking to pkg.go.dev.
