Add PUT method support to the file:// handler, allowing file uploads via
curl -T. Upload is opt-in, disabled by default — enable with ?put=true
metadata flag. Includes path traversal protection.
Fix bugs in the file handler: unbuffered send channel deadlock on Close,
conn leak when send drops on done, and Addr() returning nil. Add done-
priority select in send(), explicit ln.Close() in Close(), Unwrap() on
responseWriter for http.ResponseController, and nil guards.
Also add package doc, NewHandler doc, and 39 unit tests covering the
listener, responseWriter, handler lifecycle, auth, and file serving.
ginuerzh