fix(cr-2026-04-22): recording_filename avoids same-second collisions

MAJOR from the 2026-04-22 review (audio.rs:236-257): filenames were
derived from SystemTime::now().as_secs(), so two recordings started
within the same second resolved to the same path — possible overwrite
or merge.

Extracts the filename generation into a private helper and appends
the sub-second nanosecond component. Format is now
`kon-<secs>-<nanos_in_sec>.wav`, e.g. `kon-1776828000-123456789.wav`,
which stays human-readable, sortable by timestamp, and effectively
uncollidable under any realistic live-capture pattern.

Two tests cover the regression:
- recording_filenames_are_unique_across_rapid_calls: 64 tight-loop
  calls must produce at least 32 unique names.
- recording_filename_has_expected_shape: prefix/suffix plus the
  zero-padded 9-digit nanos component.

src-tauri/src/commands/audio.rs

@@ -249,11 +249,62 @@ pub fn resolve_recording_path(
     std::fs::create_dir_all(&recordings_dir)
         .map_err(|e| format!("Failed to create recordings dir: {e}"))?;
 
-    let timestamp = std::time::SystemTime::now()
+    Ok(recordings_dir.join(recording_filename()))
+}
+
+/// Deterministic recording filename generator. Uses seconds since the
+/// epoch for human-readable sortability plus the sub-second nanosecond
+/// component so two recordings started within the same second do not
+/// collide (brief item from the 2026-04-22 code review).
+///
+/// Format: `kon-<secs>-<nanos_in_sec>.wav`, e.g.
+/// `kon-1776828000-123456789.wav`.
+fn recording_filename() -> String {
+    let duration = std::time::SystemTime::now()
         .duration_since(std::time::UNIX_EPOCH)
-        .unwrap_or_default()
-        .as_secs();
-    Ok(recordings_dir.join(format!("kon-{timestamp}.wav")))
+        .unwrap_or_default();
+    let secs = duration.as_secs();
+    let nanos = duration.subsec_nanos();
+    format!("kon-{secs}-{nanos:09}.wav")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::recording_filename;
+
+    #[test]
+    fn recording_filenames_are_unique_across_rapid_calls() {
+        // Regression for the 2026-04-22 review: two recordings
+        // started in the same second previously collided on a
+        // seconds-only filename. The sub-second precision added
+        // here must hold under a tight loop.
+        let mut names = std::collections::HashSet::new();
+        for _ in 0..64 {
+            names.insert(recording_filename());
+        }
+        assert!(
+            names.len() >= 32,
+            "expected at least 32 unique filenames across 64 rapid calls, got {}",
+            names.len()
+        );
+    }
+
+    #[test]
+    fn recording_filename_has_expected_shape() {
+        let name = recording_filename();
+        assert!(name.starts_with("kon-"));
+        assert!(name.ends_with(".wav"));
+        // Shape: kon-<digits>-<9 digits>.wav
+        let rest = name
+            .strip_prefix("kon-")
+            .and_then(|s| s.strip_suffix(".wav"))
+            .expect("shape prefix/suffix");
+        let parts: Vec<&str> = rest.split('-').collect();
+        assert_eq!(parts.len(), 2, "expected two '-' separated parts, got {parts:?}");
+        assert!(parts[0].chars().all(|c| c.is_ascii_digit()), "secs is digits");
+        assert_eq!(parts[1].len(), 9, "nanos component is zero-padded to 9 digits");
+        assert!(parts[1].chars().all(|c| c.is_ascii_digit()));
+    }
 }
 
 pub async fn persist_audio_samples(