Return real handle metadata from Windows fstat

[LunaStev]

Windows fstat obtains only the file size, then reports a regular-file mode and mtime = 0 for every successful call. The adjacent path-based stat reads actual attributes and last-write time. Querying the same ordinary file through a path and an open handle can therefore report different modification times, even without a concurrent change.

Source evidence at the head of #520:

• std/sys/windows/fs.wave:219 — pub fun fstat
• std/sys/windows/fs.wave:229 — pub fun stat

Acceptance:

• Populate size, type and modification time from native handle metadata, with the same units as path-based stat.
• Compare stat/fstat for a nonempty temporary file with a known nonzero timestamp.
• Handle directory and non-file handles explicitly; never fabricate regular-file metadata when unsupported.
• Add native amd64 and ARM64 coverage without libc FFI.

Related: #385. Error translation is a separate concern from filling successful metadata.

Audit status: identified by static source inspection; the scenarios above have not been executed during this audit. The permalink fixes the reviewed revision; this report does not claim the defect was introduced by #520.