fix(foundation): a Windows pipe passes seekable(), so its peeked octets are lost #2097

The first Windows run of #2095's leg (job 120605026883) fails 25 pipe cases in test_extraction_non_seekable_stream_1586_unit, including pcapkit - reading from stdin. PCAP reads its magic as b'\x02\x00\x04\x00', which is the version bytes at offset 4, so the bytes the peek consumed are lost.

Inferred, not yet probed on a runner: on a Windows pipe, seekable() returns True and seek silently does nothing. The Extractor.__init__ branch then skips the SeekableReader wrapper, and PeekableStream.peek's seek-back never rewinds. A one-line probe on the leg would confirm it.

Proposed: trust seekable() only for a regular file (stat.S_ISREG(os.fstat(fileno).st_mode)), and otherwise wrap in SeekableReader. This is a user-visible defect, so it blocks #2060 phase 2.