fix: preserve errno through XS scope cleanup

[Koan-Bot]

What

Save and restore errno around FREETMPS/LEAVE in the three XS callback wrapper functions.

Why

_check() (or the user callback) sets $! to indicate the error type (ENOENT, EACCES, etc.) for failed operations. The XS scope cleanup (FREETMPS/LEAVE) can trigger SV destruction or other Perl internals that clobber the C errno value. This causes $! to be 0 instead of the expected errno after failed stat() calls on mocked files.

File checks (-e, -f) were less affected because _check_from_stat determines success via stat array length, not errno. But stat() callers that check $! after a failed stat saw incorrect values.

How

Three functions patched with the same pattern:

• _overload_ft_ops() — file check path
• _overload_ft_ops_sv() — file check returning SV path
• _overload_ft_stat() — stat/lstat path

Each saves errno before PUTBACK/FREETMPS/LEAVE and restores it after. This matches the pattern used in Perl core (e.g., pp_close).

Testing

New test file t/stat-errno-preservation.t covering:

• errno set by callback is preserved through stat/lstat/file-check operations
• Custom errno (EACCES) preserved through XS cleanup
• Default ENOENT set when callback omits errno

Fixes the smoker failure reported in cpanel/Test-MockFile#207.