Feat(rust): Implement WebVTT-specific timestamp format and layout anchor (#2135)

* Feat(rust): Implement WebVTT-specific timestamp format and layout anchor

* style: apply rustfmt to g608.rs

* fix(rust): use WebVTT-spec dot separator for milliseconds in timestamp line

Author: pranavshar223

src/rust/src/encoder/g608.rs

@@ -191,9 +191,20 @@ pub fn write_cc_buffer_as_g608(data: &eia608_screen, context: &mut encoder_ctx)
     }
 
     // Create timeline string for timestamps
-    let timestamp_line = format!(
-        "{h1:02}:{m1:02}:{s1:02},{ms1:03} --> {h2:02}:{m2:02}:{s2:02},{ms2:03}{encoded_clrf}"
-    );
+    let is_webvtt = context.write_format == crate::bindings::ccx_output_format::CCX_OF_WEBVTT;
+
+    let timestamp_line = if is_webvtt {
+        // WebVTT Standard requires dots instead of commas for milliseconds
+        // and supports trailing settings (e.g., vertical text direction, alignment)
+        format!(
+            "{h1:02}:{m1:02}:{s1:02}.{ms1:03} --> {h2:02}:{m2:02}:{s2:02}.{ms2:03}{encoded_clrf}"
+        )
+    } else {
+        // Legacy SRT Standard requires commas
+        format!(
+            "{h1:02}:{m1:02}:{s1:02},{ms1:03} --> {h2:02}:{m2:02}:{s2:02},{ms2:03}{encoded_clrf}"
+        )
+    };
 
     // Encode and write timestamp line
     let used = encode_line(context, buffer_slice, timestamp_line.as_bytes());