Logger: use fputs for stderr to fix output ordering and a closed-fd crash #390

3.0 moved Logger.error and Logger.warning to stderr via FileHandle.standardError.write. Two small problems with that choice:

1. Ordering. print writes to a block-buffered stdout when output is piped; FileHandle.write is unbuffered. Under 2>&1 — i.e. any CI log — warnings now appear before the "Report successfully created" line rather than interleaved where they belong.
2. Crash on a closed descriptor. FileHandle.write raises an uncatchable Objective-C exception, so xchtmlreport … 2>&- now crashes where it previously failed silently.

Task: use fputs(message + "\n", stderr) instead, which fixes both. Optionally fflush(stdout) before writing to stderr to keep interleaving exact.

Good first issue — a couple of lines in Logger.swift.