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.ps1.in

@@ -173,7 +173,7 @@ Register-ArgumentCompleter -Native -CommandName '{{PROJECT_EXECUTABLE}}' -Script
         #   [CompletionResult]::new('-L'                      , 'L'                     , [CompletionResultType]::ParameterName, 'Display all supported languages.')
             [CompletionResult]::new('--list-languages'        , 'list-languages'        , [CompletionResultType]::ParameterName, 'Display all supported languages.')
         #   [CompletionResult]::new('-u'                      , 'u'                     , [CompletionResultType]::ParameterName, 'u')
-            [CompletionResult]::new('--unbuffered'            , 'unbuffered'            , [CompletionResultType]::ParameterName, 'unbuffered')
+            [CompletionResult]::new('--unbuffered'            , 'unbuffered'            , [CompletionResultType]::ParameterName, 'Enable unbuffered input reading for streaming use cases')
             [CompletionResult]::new('--completion'            , 'completion'            , [CompletionResultType]::ParameterName, 'Show shell completion for a certain shell. [possible values: bash, fish, zsh, ps1]')
             [CompletionResult]::new('--no-config'             , 'no-config'             , [CompletionResultType]::ParameterName, 'Do not use the configuration file')
             [CompletionResult]::new('--no-custom-assets'      , 'no-custom-assets'      , [CompletionResultType]::ParameterName, 'Do not load custom assets')

assets/completions/bat.bash.in

@@ -190,8 +190,8 @@ _bat() {
 	$split && return 0
 
 	if [[ $cur == -* ]]; then
-		# --unbuffered excluded intentionally (no-op)
 		COMPREPLY=($(compgen -W "
+			--unbuffered
 			--show-all
 			--nonprintable-notation
 			--binary

assets/completions/bat.fish.in

@@ -239,7 +239,7 @@ complete -c $bat -l theme-dark -x -a "(command $bat --list-themes | command cat)
 
 complete -c $bat -l theme-light -x -a "(command $bat --list-themes | command cat)" -d "Set the syntax highlighting theme for light backgrounds" -n __bat_no_excl_args
 
-complete -c $bat -s u -l unbuffered -d "This option exists for POSIX-compliance reasons" -n __bat_no_excl_args
+complete -c $bat -s u -l unbuffered -d "Enable unbuffered input reading for streaming use cases" -n __bat_no_excl_args
 
 complete -c $bat -s V -l version -f -d "Show version information" -n __fish_is_first_arg
 

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]'