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:

utils/sdr/rtlsdr.py

@@ -197,6 +197,43 @@ class RTLSDRCommandBuilder(CommandBuilder):
 
         return cmd
 
+    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 rtl_sdr command for raw IQ capture.
+
+        Outputs raw uint8 IQ to stdout for processing by IQ pipelines.
+        """
+        rtl_sdr_path = get_tool_path('rtl_sdr') or 'rtl_sdr'
+        freq_hz = int(frequency_mhz * 1e6)
+        cmd = [
+            rtl_sdr_path,
+            '-d', self._get_device_arg(device),
+            '-f', str(freq_hz),
+            '-s', str(sample_rate),
+        ]
+
+        if gain is not None and gain > 0:
+            cmd.extend(['-g', str(gain)])
+
+        if ppm is not None and ppm != 0:
+            cmd.extend(['-p', str(ppm)])
+
+        if bias_t:
+            cmd.extend(['-T'])
+
+        # Output to stdout
+        cmd.append('-')
+
+        return cmd
+
     def build_ais_command(
         self,
         device: SDRDevice,