range-parser never validates the range unit, so /api/file now parses only
headers that start with the bytes unit: unsatisfiable ranges answer 416,
while malformed, unknown-unit, and multi-range headers are ignored for a
full 200 body per RFC 9110. The filesystem-root workspace case stays
POSIX-only in tests (a Windows drive-root spelling cannot be constructed
portably); containment already treats any separator-terminated root the
same. Docs (README pair, Agent Note pair) re-recorded.