fix(portal): convert PipeWire nanosecond PTS to encoder frame-number units

PipeWire spa_meta_header.pts is CLOCK_MONOTONIC in nanoseconds, but the
encoder expects frame-number units (time_base = 1/fps). The raw nanosecond
value was assigned directly to AVFrame.pts, causing the encoder/muxer to
interpret timestamps as billions of frames, producing corrupted duration
metadata and broken rate control.

Fix: record the first frame's PTS as a nanosecond base, compute elapsed
nanoseconds for each subsequent frame, then convert to frame numbers via
elapsed_ns * fps / 1_000_000_000. Using elapsed time avoids i64 overflow
on absolute timestamps (~10^18 ns).

Matches the WLR path pattern (state.rs:525-527) which converts microseconds
to frame numbers for the same encoder.

src/state_portal.rs

@@ -25,6 +25,7 @@ pub struct StatePortal {
     errored: bool,
     first_frame: bool,
     drm_device: PathBuf,
+    first_pts_ns: Option<i64>,
 }
 
 impl StatePortal {
@@ -43,6 +44,7 @@ impl StatePortal {
             errored: false,
             first_frame: true,
             drm_device,
+            first_pts_ns: None,
         })
     }
 
@@ -174,9 +176,16 @@ impl StatePortal {
             bail!("av_hwframe_transfer_data failed: error {ret}");
         }
 
-        // 7. Set PTS
+        // 7. Set PTS — convert PipeWire nanoseconds to encoder frame-number units
+        //    PipeWire PTS is CLOCK_MONOTONIC in nanoseconds.
+        //    Encoder time_base = 1/fps, so PTS must be in frame numbers.
+        //    Use elapsed time since first frame to avoid i64 overflow on absolute timestamps.
+        let fps_i64 = self.args.fps as i64;
+        let base_ns = *self.first_pts_ns.get_or_insert(frame.pts.max(0));
+        let elapsed_ns = (frame.pts.max(0) - base_ns).max(0);
+        let pts = elapsed_ns * fps_i64 / 1_000_000_000;
         unsafe {
-            (*hw_frame.as_mut_ptr()).pts = frame.pts;
+            (*hw_frame.as_mut_ptr()).pts = pts;
         }
 
         // 8. Encode