feat: implement --unbuffered mode for streaming input (#3555)

Repurpose the existing --unbuffered/-u flag (previously a POSIX no-op)
to enable unbuffered input reading using fill_buf()/consume() instead
of read_until(b'\n'). This allows partial lines to display immediately
when piping streaming input like `tail -f` into bat.

- Add unbuffered field to Config and InputReader
- Add read_line_unbuffered() using BufRead::fill_buf()/consume()
- Add flush() to OutputHandle, called after each line in unbuffered mode
- Auto-disable line numbers in unbuffered mode to avoid partial line confusion
- Update help text, man page, and shell completions
- Add unit tests and integration tests

assets/completions/bat.zsh.in

@@ -58,7 +58,7 @@ _{{PROJECT_EXECUTABLE}}_main() {
             default auto full plain changes header header-filename header-filesize grid rule numbers snip'
         \*{-r+,--line-range=}'[only print the specified line range]:start\:end'
         '(* -)'{-L,--list-languages}'[display all supported languages]'
-        '(-u --unbuffered)'--unbuffered'[this option exists for POSIX-compliance reasons]'
+        '(-u --unbuffered)'--unbuffered'[enable unbuffered input reading for streaming use cases]'
         --completion='[show shell completion for a certain shell]:shell:(bash fish zsh ps1)'
         --set-terminal-title'[sets terminal title to filenames when using a pager]'
         --diagnostic'[show diagnostic information for bug reports]'