secondary-sdr: fix ADS-B on real hardware (tested on radio-box)

First hardware test of node-26#9/#10 found three blockers:
- antirez/dump1090 has no --write-json, so ADS-B mode exited on start.
  Swapped to wiedehopf/readsb; map alt_baro/gs (old names as fallback).
- op25 is not always on RTL-SDR index 0 (radio-box: op25 on 1, 0 free).
  start() now tries each index and keeps the first decoder that stays up.
  AIS-catcher index flag fixed to -d:N ("-d N" selects by serial).
- status() reported "running" for a decoder that died on startup: the
  zombie still answered killpg(pgid, 0). Liveness now via Popen.poll().

install.sh blacklists dvb_usb_rtl28xxu, which claimed the second dongle.

Verified on radio-box: 831 msgs/min, 6 aircraft (3 with position) on a
9cm whip; op25 unaffected. AIS still untested.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

secondary-sdr-container/Dockerfile

@@ -23,12 +23,14 @@ RUN apt-get update && \
     apt-get upgrade -y && \
     apt-get install -y --no-install-recommends \
         git build-essential cmake pkg-config \
-        librtlsdr-dev libusb-1.0-0-dev libssl-dev zlib1g-dev usbutils
+        librtlsdr-dev libusb-1.0-0-dev libssl-dev zlib1g-dev libzstd-dev libncurses-dev usbutils
 
-# dump1090 (antirez/classic) — ADS-B decoder. --write-json support is a
-# long-standing, well-documented feature of this fork.
-RUN git clone https://github.com/antirez/dump1090 /opt/dump1090 && \
-    cd /opt/dump1090 && make
+# readsb (wiedehopf) — ADS-B decoder. antirez/dump1090 was used here first but
+# has no --write-json at all (it only serves /data.json over --net), so the
+# decoder exited on an unknown flag. readsb writes the dump1090-fa style
+# aircraft.json that _read_adsb_snapshot() parses.
+RUN git clone --depth 1 https://github.com/wiedehopf/readsb /opt/readsb && \
+    cd /opt/readsb && make RTLSDR=yes
 
 # AIS-catcher — AIS decoder.
 RUN git clone https://github.com/jvde-github/AIS-catcher /opt/AIS-catcher && \