chore: clippy and parser-style cleanups (no behavior change)

Three small drive-by cleanups picked up by clippy on touched files; none
changes behavior.

src/backend/whisper_diarization.rs:
src/tests/conformance_harness.rs (parse_srt_time_for_fixture):
- Replace the `else if let Some(pos) = value.rfind('.') { ... } else { return None; }`
  branch with `let pos = value.rfind('.')?;` collapsing the fallback
  millisecond-separator handling into a single ? returning None.
  Both copies of the SRT timestamp parser now match.

tests/metamorphic_audio_tests.rs:
- Annotate the silence-generator helper `generate_silence` with
  `#[allow(dead_code)]` so the no-op metamorphic case (which leaves
  silence generation for a future scenario) compiles without a warning.
- Replace `std::iter::repeat(0.0).take(n)` with `std::iter::repeat_n(0.0, n)`
  per clippy::manual_repeat_n on Rust 1.82+.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Dicklesworthstone

src/backend/whisper_diarization.rs

@@ -308,10 +308,9 @@ fn parse_srt_time(value: &str) -> Option<f64> {
     // Support both standard ',' and fallback '.' millisecond separator.
     let (hms, ms_str) = if let Some(pos) = value.rfind(',') {
         (&value[..pos], &value[pos + 1..])
-    } else if let Some(pos) = value.rfind('.') {
-        (&value[..pos], &value[pos + 1..])
     } else {
-        return None;
+        let pos = value.rfind('.')?;
+        (&value[..pos], &value[pos + 1..])
     };
 
     let ms = ms_str.parse::<f64>().ok()?;

tests/conformance_harness.rs

@@ -693,10 +693,9 @@ fn parse_srt_block_for_fixture(block: &str) -> Option<TranscriptionSegment> {
 fn parse_srt_time_for_fixture(value: &str) -> Option<f64> {
     let (hms, ms_str) = if let Some(pos) = value.rfind(',') {
         (&value[..pos], &value[pos + 1..])
-    } else if let Some(pos) = value.rfind('.') {
-        (&value[..pos], &value[pos + 1..])
     } else {
-        return None;
+        let pos = value.rfind('.')?;
+        (&value[..pos], &value[pos + 1..])
     };
 
     let mut hms_parts = hms.split(':');

tests/metamorphic_audio_tests.rs

@@ -32,6 +32,7 @@ fn generate_sine_wave(freq_hz: f32, sample_rate: u32, duration_secs: f32) -> Vec
 }
 
 /// Generate silence (zeros) for the given duration.
+#[allow(dead_code)]
 fn generate_silence(sample_rate: u32, duration_secs: f32) -> Vec<f32> {
     let num_samples = (sample_rate as f32 * duration_secs) as usize;
     vec![0.0; num_samples]
@@ -70,7 +71,7 @@ fn prepend_silence(samples: &[f32], silence_samples: usize) -> Vec<f32> {
 /// Append silence to audio.
 fn append_silence(samples: &[f32], silence_samples: usize) -> Vec<f32> {
     let mut result = samples.to_vec();
-    result.extend(std::iter::repeat(0.0).take(silence_samples));
+    result.extend(std::iter::repeat_n(0.0, silence_samples));
     result
 }