feat(stats): expose duplicate_frames_skipped counter (closes #20)

Final piece of #20. The EncodeOutcome::SkippedDuplicate variant was
introduced in #19 but its count was invisible — silent Ok(_) arm in
encode_thread_loop. Now exposed as a stat.

Changes:

- stats.rs: PipelineStats gains duplicate_frames_skipped (window delta)
  and prev_duplicate_frames_skipped (running total). Snapshot field
  added. Display format places it after over_budget (both are counters).
  Reset clears window delta but preserves running total (same pattern
  as pipewire_dropped).

- state_portal.rs: EncodeThread struct gains duplicate_count:
  Arc<AtomicU64>. Cloned for encode_thread_loop, stored for main-thread
  reads. encode_thread_loop now explicitly matches SkippedDuplicate and
  increments with Ordering::Relaxed. Stats snapshot code reads atomic
  and calls set_duplicate_frames_skipped after timing drain.

What this enables:

Diagnosing encoded_fps health. Examples:
  - capture_fps=60 encoded_fps=30 duplicate_frames_skipped=30
    → healthy: encoder at 30fps target, 30 frames were true duplicates
  - capture_fps=60 encoded_fps=5 duplicate_frames_skipped=0
    → problem: frames not being dedup'd but encoder can't keep up
  - capture_fps=1.7 encoded_fps=1.7 duplicate_frames_skipped=0
    → healthy static: low fps because KWin damage-driven delivery

Original #20 issues status:
  - 'encoded_fps stuck at ~30': FIXED via #19 (EncodeOutcome enum), now
    tracks capture_fps when below 30
  - 'filler masking real fps': FIXED via #15/#18 (filler deleted)
  - 'duplicate count invisible': FIXED via this commit
  - 'unique_encoded_fps / delivered_fps': not implemented, deemed
    unnecessary now that the core metrics are trustworthy

Tests:
- cargo build --release: 0 new warnings (19 baseline preserved)
- cargo test: 96 lib + 3 integration, 0 failed
- SAFETY comments preserved verbatim
- 2 files changed, +45/-6 lines

src/stats.rs

@@ -39,6 +39,12 @@ pub struct PipelineStats {
     sent_frames: u64,
     pipewire_dropped: u64,
     over_budget_count: u64,
+    /// Count of frames dropped by encode thread due to Y-plane hash dedup
+    /// (EncodeOutcome::SkippedDuplicate). Read from atomic counter set by
+    /// encode thread, computed as delta since previous snapshot.
+    duplicate_frames_skipped: u64,
+    // Running total from the encode-thread atomic; NOT reset between windows.
+    prev_duplicate_frames_skipped: u64,
 
     // --- queue depth at last observation ---
     capture_queue_depth: usize,
@@ -75,6 +81,8 @@ impl PipelineStats {
             sent_frames: 0,
             pipewire_dropped: 0,
             over_budget_count: 0,
+            duplicate_frames_skipped: 0,
+            prev_duplicate_frames_skipped: 0,
             capture_queue_depth: 0,
             encoded_queue_depth: 0,
             capture_gaps_ms: Vec::new(),
@@ -186,6 +194,13 @@ impl PipelineStats {
         self.pipewire_dropped = total_dropped.saturating_sub(prev_dropped);
     }
 
+    /// Update duplicate frames skipped counter (absolute value from atomic).
+    /// Computes delta from previous value, like set_pipewire_dropped.
+    pub fn set_duplicate_frames_skipped(&mut self, total_skipped: u64) {
+        self.duplicate_frames_skipped = total_skipped.saturating_sub(self.prev_duplicate_frames_skipped);
+        self.prev_duplicate_frames_skipped = total_skipped;
+    }
+
     /// Update queue depth observations.
     pub fn set_queue_depths(&mut self, capture: usize, encoded: usize) {
         self.capture_queue_depth = capture;
@@ -216,6 +231,7 @@ impl PipelineStats {
             sent_frames: self.sent_frames,
             pipewire_dropped: self.pipewire_dropped,
             over_budget_count: self.over_budget_count,
+            duplicate_frames_skipped: self.duplicate_frames_skipped,
             capture_queue_depth: self.capture_queue_depth,
             encoded_queue_depth: self.encoded_queue_depth,
             capture_gap_avg_ms: avg_f64(&self.capture_gaps_ms),
@@ -254,6 +270,7 @@ impl PipelineStats {
         self.sent_frames = 0;
         self.pipewire_dropped = 0;
         self.over_budget_count = 0;
+        self.duplicate_frames_skipped = 0;
         self.capture_queue_depth = 0;
         self.encoded_queue_depth = 0;
         self.capture_gaps_ms.clear();
@@ -288,6 +305,7 @@ pub struct StatsSnapshot {
     pub sent_frames: u64,
     pub pipewire_dropped: u64,
     pub over_budget_count: u64,
+    pub duplicate_frames_skipped: u64,
     // Queue depths
     pub capture_queue_depth: usize,
     pub encoded_queue_depth: usize,
@@ -330,8 +348,8 @@ impl std::fmt::Display for StatsSnapshot {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(
             f,
-            "capture_fps={:.1} encoded_fps={:.1} sent_fps={:.1} \
-             pw_dropped={} over_budget={} \
+             "capture_fps={:.1} encoded_fps={:.1} sent_fps={:.1} \
+             pw_dropped={} over_budget={} duplicate_frames_skipped={} \
              cap_q={} enc_q={} \
              cap_gap_p95={:.1}ms cap_gap_max={:.1}ms \
              enc_gap_p95={:.1}ms enc_gap_max={:.1}ms \
@@ -346,6 +364,7 @@ impl std::fmt::Display for StatsSnapshot {
             self.sent_fps,
             self.pipewire_dropped,
             self.over_budget_count,
+            self.duplicate_frames_skipped,
             self.capture_queue_depth,
             self.encoded_queue_depth,
             self.capture_gap_p95_ms,