fix: share soundcard between darkice and ISR via ALSA dsnoop

hw:0,0 is an exclusive ALSA device — darkice holding it caused arecord
to fail silently (stderr was /dev/null), leaving all recordings at 0 bytes
with no errors in the log.

asound.conf: defines a dsnoop virtual device 'shared_mic' that opens
hw:0,0 once and lets multiple processes capture simultaneously.

docker-compose.yml: mount asound.conf into the container as
/etc/asound.conf; add ipc: host so the container shares the host IPC
namespace (dsnoop uses System V shared memory which does not cross the
container IPC boundary without this).

config.example.ini: document the dsnoop setup and shared-device pattern.
README, CLAUDE.md: document the full setup procedure.

config.example.ini

@@ -144,3 +144,19 @@ format = auto
 # sample_rate = 44100
 # channels = 2
 # format = flac
+#
+# SHARING A DEVICE WITH ANOTHER APP (e.g. darkice):
+# ALSA hw: devices are exclusive — only one process can open them at a time.
+# Use the dsnoop virtual device defined in asound.conf to share the hardware:
+#
+#   1. sudo cp asound.conf /etc/asound.conf   (once, on the host)
+#   2. Change the other app (darkice etc.) to use device "shared_mic" too
+#   3. Use device = shared_mic in ISR config (docker-compose.yml mounts asound.conf automatically)
+#
+# [usb_mic_shared]
+# type = soundcard
+# device = shared_mic
+# backend = alsa
+# sample_rate = 48000
+# channels = 2
+# format = flac