Handle broken pipe on Windows (non-Unix) gracefully

[Mathjk]

Fixes #1068

Summary

reset_sigpipe is a no-op on Windows (#[cfg(unix)]), so when the
process reading difftastic's stdout exits early — difft a b | head -2 —
every println! path panics:

thread 'main' panicked at library/std/src/io/stdio.rs:
failed printing to stdout: The pipe is being closed. (os error 232)

On Unix the same invocation dies quietly to SIGPIPE (exit 141). This PR
makes the non-Unix path match that behavior.

Approach

print!/println! panic inside std on write errors, so there is no
interceptable choke point. Instead this threads a single
&mut impl io::Write (a StdoutLock owned by main) through the
~47 output sites across the display and parse modules, all now returning
io::Result<()>:

• main() splits into run(mode, out) -> io::Result<()>; the
  wrapper maps ErrorKind::BrokenPipe to process::exit(EXIT_SUCCESS)
  and keeps the identical panic message for other write errors
  (parity with std's failed printing to stdout message).
• A plain StdoutLock is used (not BufWriter) so process::exit
  paths can never lose buffered output. As a side benefit there is now
  one lock acquisition per result instead of per line.
• Latent fix: the parallel dir-diff sender thread did
  .expect(\"Receiver should be connected\"); once the receiver can
  drop early (on a ? bail) that would panic. It now ignores the send
  error with an explanatory comment — only reachable when we are already
  erroring out.

stderr output (eprintln!) is intentionally unchanged — panic on a
closed stderr is the status quo on all platforms.

Verification

• Every output mode tested against a reader that exits immediately:
  side-by-side, inline, --display=json, --dump-syntax,
  --dump-ts, --list-languages, dir diffs, git unmerged mode —
  all quiet exit 0 where they previously panicked (verified by
  neutralizing the SIGPIPE reset so the Unix build takes the Windows
  code path exactly).
• Real Windows binary (x86_64-pc-windows-gnu, run on Windows 11):
  difft a.py b.py | Select-Object -First 0 and -First 2 exit 0;
  normal output unchanged.
• Output is byte-identical to master across 14 captures (side-by-side,
  inline, json, dump modes, list-languages, exit codes).
• cargo fmt --check clean; clippy shows only pre-existing warnings;
  cargo test: 123 unit + 24 CLI tests pass; cargo check --target x86_64-pc-windows-gnu clean.

Disclosure

Implemented with AI assistance (Devin); the change was reviewed by the
account owner, and verified as described above. See issue #1068.