Add live waterfall during pager and sensor decoding via IQ pipeline

Replace rtl_fm/rtl_433 with rtl_sdr for raw IQ capture when available,
enabling a Python IQ processor to compute FFT for the waterfall while
simultaneously feeding decoded data to multimon-ng (pager) or rtl_433
(sensor). Falls back to the legacy pipeline when rtl_sdr is unavailable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

utils/sdr/base.py

@@ -186,6 +186,36 @@ class CommandBuilder(ABC):
         """Return hardware capabilities for this SDR type."""
         pass
 
+    def build_raw_capture_command(
+        self,
+        device: SDRDevice,
+        frequency_mhz: float,
+        sample_rate: int,
+        gain: Optional[float] = None,
+        ppm: Optional[int] = None,
+        bias_t: bool = False
+    ) -> list[str]:
+        """
+        Build raw IQ capture command (for IQ-based waterfall during decoding).
+
+        Args:
+            device: The SDR device to use
+            frequency_mhz: Center frequency in MHz
+            sample_rate: Sample rate in Hz
+            gain: Gain in dB (None for auto)
+            ppm: PPM frequency correction
+            bias_t: Enable bias-T power (for active antennas)
+
+        Returns:
+            Command as list of strings for subprocess
+
+        Raises:
+            NotImplementedError: If the SDR type does not support raw capture
+        """
+        raise NotImplementedError(
+            f"Raw IQ capture not supported for {self.get_sdr_type().value}"
+        )
+
     @classmethod
     @abstractmethod
     def get_sdr_type(cls) -> SDRType: