`goclaw migrate up` (and every other migrate subcommand) failed on Windows
with `create migrator: failed to open source, "file:///D:/.../migrations":
open .: The filename, directory name, or volume label syntax is incorrect.`
golang-migrate's file source driver mis-parses absolute drive-letter file://
URLs; the drive-letter formatting in absoluteToFileURI produced a URL the
driver could not open. Replace the file:// URL with an iofs source over
os.DirFS, which uses native OS path handling and works identically on every
platform. Drop the now-unused absoluteToFileURI/migrationsSourceURL helpers
and their URL-shape tests, and add a DB-free regression test that opens the
real migrations directory via newMigrationSource().
Verified end-to-end on Windows: `migrate version` and `migrate up` now run.
golang-migrate's file source driver parsed "file://F:\\project\\..."
with "F" as host and ":\\..." as port → "invalid port" error, so
upgrade/migrate commands crashed on Windows before applying any SQL.
Format the source URL per RFC 8089: forward-slash separators and a
leading "/" before the drive letter → "file:///F:/project/...".
POSIX paths pass through unchanged (leading "/" already present).
Shape-driven, not OS-driven: no runtime.GOOS branch needed, which
also lets the unit test cover both shapes on a single runner.
Closes#872
thotam