API changes for v1:
- Collect(fsys, subpath) takes a subdirectory path (use "." for root),
enabling embed.FS with //go:embed sql/migrations/*.sql
- Latest() applies all pending migrations (shorthand for Up with n=-1)
- Drop() rolls back all applied migrations (shorthand for Down with n=-1)
os.IsNotExist does not recognize fs.ErrNotExist when wrapped by an
fs.FS implementation. Switch to errors.Is(err, fs.ErrNotExist) so
the "file not found" check works for both os.Open and fs.FS.Open.
Passing 0 to Up() or Down() is an easy mistake — it silently means
"all" which could be destructive. Now n=0 returns ErrInvalidN.
Convention: n > 0 for a specific count, n < 0 (typically -1) for all.
Factor the inline migration logic from cmd/sql-migrate into reusable
packages: database/sqlmigrate (core types, matching, file collection)
and database/sqlmigrate/shmigrate (shell script generation backend).
No behavior changes — the CLI produces identical output. The shmigrate
package implements the sqlmigrate.Migrator interface so other backends
(pgmigrate, mymigrate, etc.) can follow the same pattern.
