Add --fallback-syntax for undetected files (#3617)

* feat(cli): add fallback syntax option

Expose a new fallback syntax CLI option so users can opt into syntax highlighting only when auto-detection fails.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

* feat(syntax): apply fallback only after detection fails

Use the fallback syntax only when path and first-line detection fail, preserving existing behavior for detected files and explicit language selection.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

* test(cli): cover fallback syntax behavior

Add integration coverage for fallback syntax usage, precedence with --language, and no-op behavior when syntax is already detected; update help snapshots for the new option.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

* docs(changelog): document fallback syntax option

Record the new fallback syntax feature in the unreleased changelog section.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

---------

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/bin/bat/app.rs

@@ -384,6 +384,10 @@ impl App {
                         None
                     }
                 }),
+            fallback_syntax: self
+                .matches
+                .get_one::<String>("fallback-syntax")
+                .map(|s| s.as_str()),
             show_nonprintable: self.matches.get_flag("show-all"),
             nonprintable_notation: match self
                 .matches

src/bin/bat/clap_app.rs

@@ -120,6 +120,17 @@ pub fn build_app(interactive_output: bool) -> Command {
                      language names and file extensions.",
                 ),
         )
+        .arg(
+            Arg::new("fallback-syntax")
+                .long("fallback-syntax")
+                .visible_alias("fallback-language")
+                .help("Set a fallback language for undetected syntaxes.")
+                .long_help(
+                    "Set a fallback language for syntax highlighting when auto-detection fails. \
+                     Unlike '--language', this is only used when no syntax could be detected from \
+                     filename, custom syntax mappings, or first-line detection.",
+                ),
+        )
         .arg(
             Arg::new("highlight-line")
                 .long("highlight-line")