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

doc/long-help.txt

@@ -203,8 +203,11 @@ Options:
           Display a list of supported languages for syntax highlighting.
 
   -u, --unbuffered
-          This option exists for POSIX-compliance reasons ('u' is for 'unbuffered'). The output is
-          always unbuffered - this option is simply ignored.
+          Enable unbuffered input reading. When this flag is set, bat will display data as soon as
+          it is available, without waiting for a complete line. This is useful for streaming use
+          cases like 'tail -f logfile | bat -u --paging=never'. Note that line numbers are
+          automatically disabled in unbuffered mode, and syntax highlighting may be imperfect on
+          partial lines.
 
       --completion <SHELL>
           Show shell completion for a certain shell. [possible values: bash, fish, zsh, ps1]

doc/short-help.txt

@@ -58,6 +58,8 @@ Options:
           Only print the lines from N to M.
   -L, --list-languages
           Display all supported languages.
+  -u, --unbuffered
+          Enable unbuffered input reading for streaming use cases.
       --completion <SHELL>
           Show shell completion for a certain shell. [possible values: bash, fish, zsh, ps1]
   -E, --quiet-empty