Fix panic in process_page() on negative teletext PTS timestamps (#2240)

show_timestamp.to_srt_time().expect() and hide_timestamp.to_srt_time().expect()
in TeletextContext::process_page() panicked for any negative Timestamp value.
Negative timestamps are common in broadcast captures with wrap-around or
uninitialized PTS — crashing after potentially processing an entire file.

to_srt_time() → as_hms_millis() → i64::try_into::<u64>() returns
OutOfRangeError for negative values; .expect() made this fatal.

Fix: process_page() already returns Option<Subtitle>, so replace both
.expect() calls with .ok()? — silently skipping the subtitle when the
timestamp is out of range, matching the function's existing None-on-empty
contract.

Fixes #2233

src/rust/lib_ccxr/src/teletext.rs

@@ -1003,16 +1003,17 @@ impl<'a> TeletextContext<'a> {
 
         let mut line_count: u8 = 0;
         let mut time_reported = false;
+        // Negative timestamps occur with wrap-around/uninitialized PTS in broadcast captures
         let timecode_show = self
             .page_buffer
             .show_timestamp
             .to_srt_time()
-            .expect("could not format to SRT time");
+            .ok()?;
         let timecode_hide = self
             .page_buffer
             .hide_timestamp
             .to_srt_time()
-            .expect("could not format to SRT time");
+            .ok()?;
 
         // process data
         for row in 1..25 {