midiviz: force the spectro tap onto the sink monitor

pw-record --target <sink> on PipeWire 1.0.5 ignored the sink and
autoconnected to the default source, so the spectrogram rendered the
room's microphone instead of the rig. stream.capture.sink=true verified
live to link pw-record:input to the target sink's monitor.

tools/bridge/midiviz.py

@@ -1201,7 +1201,16 @@ class SpectrumSource:
 
     # ── capture (the subprocess half) ──────────────────────────────────────
     def _cmd(self, target: str) -> list[str]:
-        return ["pw-record", "--target", target, "--rate", str(self.rate),
+        # `--target <sink>` alone is not enough on PipeWire 1.0.5: the capture
+        # stream ignored the sink and autoconnected to the default SOURCE, so
+        # the spectrograph rendered the room — keyboard clacks included —
+        # instead of the rig (proven on the live graph 2026-11-01:
+        # pw-record:input <- sofsoundwire_4__source). `stream.capture.sink`
+        # forces the monitor of the target sink; verified live that with it
+        # pw-record:input <- <sink>:monitor_FL/FR.
+        return ["pw-record", "--target", target,
+                "-P", "{ stream.capture.sink = true }",
+                "--rate", str(self.rate),
                 "--channels", "1", "--format", "f32", "--latency", "100ms",
                 *pw_record_raw_flag(), "-"]