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
