fix(cr-2026-04-22): recording_filename uses atomic counter for absolute uniqueness

MINOR from the batch review of 6e9ed99: SystemTime::now() alone
cannot guarantee uniqueness under tight loops — two calls in the
same clock tick can return identical secs + nanos on some OS
timing resolutions. The filename reduction from "every second"
to "every nanosecond" addresses the flagged bug but leaves a
theoretical gap.

Adds a process-lifetime AtomicU64 counter, zero-padded to 4 digits,
as the third filename component. New shape:
  kon-<secs>-<nanos_in_sec>-<counter>.wav
  e.g. kon-1776828000-123456789-0000.wav

Across process restarts the counter resets to 0, but the wall-clock
secs/nanos have advanced — no cross-launch collisions possible.
Within a single process, the counter guarantees uniqueness regardless
of clock behaviour.

Test strengthened from ">=32 of 64 unique" (probabilistic) to
"1024 of 1024 unique" (absolute).

src-tauri/src/commands/audio.rs

@@ -252,40 +252,57 @@ pub fn resolve_recording_path(
     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).
+/// Deterministic recording filename generator. Combines three fields
+/// for absolute uniqueness across rapid calls:
 ///
-/// Format: `kon-<secs>-<nanos_in_sec>.wav`, e.g.
-/// `kon-1776828000-123456789.wav`.
+/// - wall-clock seconds since the epoch — human-readable and
+///   sortable;
+/// - the sub-second nanosecond component — defeats same-second
+///   collisions;
+/// - a process-lifetime atomic counter — defeats even same-nanosecond
+///   collisions, which `SystemTime::now()` alone cannot guarantee
+///   (two calls in the same clock tick can return identical nanos).
+///
+/// Format: `kon-<secs>-<nanos_in_sec>-<counter>.wav`, e.g.
+/// `kon-1776828000-123456789-0000.wav`.
 fn recording_filename() -> String {
     let duration = std::time::SystemTime::now()
         .duration_since(std::time::UNIX_EPOCH)
         .unwrap_or_default();
     let secs = duration.as_secs();
     let nanos = duration.subsec_nanos();
-    format!("kon-{secs}-{nanos:09}.wav")
+    let counter = RECORDING_COUNTER.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+    format!("kon-{secs}-{nanos:09}-{counter:04}.wav")
 }
 
+/// Process-lifetime monotonic counter for `recording_filename`. Starts
+/// at 0 on each Kon launch; wall-clock secs/nanos still advance across
+/// restarts, so cross-launch collisions are already impossible — the
+/// counter is the last-mile guarantee against within-launch same-tick
+/// collisions.
+static RECORDING_COUNTER: std::sync::atomic::AtomicU64 =
+    std::sync::atomic::AtomicU64::new(0);
+
 #[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.
+        // Regression for the 2026-04-22 review AND the review-of-
+        // review MINOR: SystemTime::now() alone cannot guarantee
+        // uniqueness under tight loops on every OS clock resolution,
+        // so the filename now includes a process-lifetime atomic
+        // counter. With the counter, uniqueness is absolute across
+        // any number of in-process calls.
         let mut names = std::collections::HashSet::new();
-        for _ in 0..64 {
+        for _ in 0..1024 {
             names.insert(recording_filename());
         }
-        assert!(
-            names.len() >= 32,
-            "expected at least 32 unique filenames across 64 rapid calls, got {}",
-            names.len()
+        assert_eq!(
+            names.len(),
+            1024,
+            "every filename must be unique (counter-backed guarantee)"
         );
     }
 
@@ -294,16 +311,22 @@ mod tests {
         let name = recording_filename();
         assert!(name.starts_with("kon-"));
         assert!(name.ends_with(".wav"));
-        // Shape: kon-<digits>-<9 digits>.wav
+        // Shape: kon-<digits>-<9 digits>-<>=4 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_eq!(
+            parts.len(),
+            3,
+            "expected three '-' 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()));
+        assert!(parts[2].len() >= 4, "counter component is zero-padded to >=4 digits");
+        assert!(parts[2].chars().all(|c| c.is_ascii_digit()));
     }
 }