feat(stats): populate frame_age metric for WebRTC path (partial #20)

Add quantitative capture-to-send latency measurement so we can diagnose
remaining latency sources after #24/#25 PTS fixes.

Previously frame_age_p95 was always 0.0ms because the WebRTC code path
never propagated capture timestamps, even though stats.rs already
supported the metric. The infrastructure existed but was disconnected.

Changes:

- avhw.rs: Add capture_time: Instant field to CpuNv12Frame (set when
  PipeWire delivers frame) and EncodedH264Frame (propagated through
  encode thread via new last_capture_time side-channel on SwEncEncode).

- state_portal.rs: Change sent_gap channel type from Sender<f64> to
  Sender<(f64, Option<f64>)> so WebRTC thread can send pre-computed
  age_ms = capture_time.elapsed() at the exact send moment (not at
  stats drain time, which would inflate the measurement by ~1s).

- stats.rs: record_send_from_thread now accepts Option<f64> age_ms
  and pushes to frame_age_ms Vec when Some.

After this commit:
- stats: log lines show real frame_age_p95 / frame_age_max in ms
- Expected range: 5-30ms (import + scale + encode + channel send)
- If much higher: server pipeline has queueing issue
- If low but user still sees latency: confirms bottleneck is network
  or browser-side (jitter buffer, decode queue)

Scope notes:

- Only Portal/PipeWire path is instrumented. wlr-screencopy path uses
  different code path (EncState, not SwEncState) and will continue to
  report frame_age=0.0ms. Adding wlr instrumentation is separate scope.

- This is diagnostic only — does NOT change user-visible behavior.
  No encoding, sending, or stats output format changes.

Tests:
- cargo build --release: 0 new warnings (19 baseline preserved)
- cargo test: 96 lib + 3 integration, 0 failed
- SAFETY comments preserved verbatim
- 3 files changed, +39/-10 lines

Refs #20.

src/stats.rs

@@ -168,13 +168,17 @@ impl PipelineStats {
 
     /// Record a frame sent from a background WebRTC thread.
     /// `gap_ms` is the pre-computed time since the previous send (0.0 = first frame).
-    /// Unlike `record_send`, this does not sample `Instant::now()`, so it remains
-    /// accurate even when batch-drained at stats snapshot time.
-    pub fn record_send_from_thread(&mut self, gap_ms: f64) {
+    /// `age_ms` is the pre-computed capture-to-send latency (None if unavailable).
+    /// Both are pre-computed on the sending thread to remain accurate when
+    /// batch-drained at stats snapshot time on the main thread.
+    pub fn record_send_from_thread(&mut self, gap_ms: f64, age_ms: Option<f64>) {
         if gap_ms > 0.0 {
             self.sent_gaps_ms.push(gap_ms);
         }
         self.sent_frames += 1;
+        if let Some(age) = age_ms {
+            self.frame_age_ms.push(age);
+        }
     }
 
     /// Update PipeWire dropped counter (absolute value from AtomicU64).