fix: detect bias-t support before passing -T to rtl_sdr/rtl_fm

Stock rtl-sdr packages don't support the -T bias-tee flag (only
RTL-SDR Blog builds do). Passing -T to stock rtl_sdr causes an
immediate exit, breaking meteor scatter and waterfall modes.

Now probes the tool's --help output before adding -T, with a regex
that avoids false-matching "DVB-T" in the description text.

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

utils/sdr/rtlsdr.py

@@ -8,6 +8,7 @@ with existing RTL-SDR installations. No SoapySDR dependency required.
 from __future__ import annotations
 
 import logging
+import re
 import subprocess
 from typing import Optional
 
@@ -23,6 +24,28 @@ def _rtl_fm_demod_mode(modulation: str) -> str:
     return 'wbfm' if mod == 'wfm' else mod
 
 
+def _rtl_tool_supports_bias_t(tool_path: str) -> bool:
+    """Check if an rtl_* tool (rtl_fm, rtl_sdr) supports the -T bias-tee flag.
+
+    The -T flag is only available in RTL-SDR Blog builds, not in stock
+    rtl-sdr packages shipped by most distros.
+    """
+    try:
+        result = subprocess.run(
+            [tool_path, '--help'],
+            capture_output=True,
+            text=True,
+            timeout=5
+        )
+        help_text = result.stdout + result.stderr
+        # Match "-T" as a CLI flag (e.g. "[-T]" or "-T enable bias"),
+        # not as part of "DVB-T" or similar text.
+        return bool(re.search(r'(?<!\w)-T\b', help_text))
+    except Exception as e:
+        logger.warning(f"Could not detect bias-t support for {tool_path}: {e}")
+        return False
+
+
 def _get_dump1090_bias_t_flag(dump1090_path: str) -> Optional[str]:
     """Detect the correct bias-t flag for the installed dump1090 variant.
 
@@ -126,7 +149,10 @@ class RTLSDRCommandBuilder(CommandBuilder):
                 cmd.extend(['-E', 'direct2'])
 
         if bias_t:
-            cmd.extend(['-T'])
+            if _rtl_tool_supports_bias_t(rtl_fm_path):
+                cmd.append('-T')
+            else:
+                logger.warning("Bias-t requested but rtl_fm does not support -T (RTL-SDR Blog drivers required).")
 
         # Output to stdout for piping
         cmd.append('-')
@@ -283,7 +309,10 @@ class RTLSDRCommandBuilder(CommandBuilder):
             cmd.extend(['-p', str(ppm)])
 
         if bias_t:
-            cmd.append('-T')
+            if _rtl_tool_supports_bias_t(rtl_sdr_path):
+                cmd.append('-T')
+            else:
+                logger.warning("Bias-t requested but rtl_sdr does not support -T (RTL-SDR Blog drivers required).")
 
         # Output to stdout
         cmd.append('-')