radio/vrtsim: Add CIRDB support for precomputed channel taps

Integrate Channel Impulse Response Database (CIRDB) to enable playback
of precomputed realistic channel taps in vrtsim, with runtime selection
via YAML metadata.

Features:
- Read precomputed channel taps from binary database with YAML sidecar
- Runtime enable via --vrtsim.cirdb with optional paths (cirdb_yaml, cirdb_file, cirdb_path)
- Channel parameter configuration: model_id (TDL-A..E), delay spread (ds_ns), UE speed (speed_mps)
- Fallback to existing channel model options when disabled
- Time-driven snapshot progression based on elapsed simulation time

Fixes:
- RNG initialization: Add randominit() call before gaussZiggurat()
- FlatBuffers linking: Handle both 'FlatBuffers' and 'flatbuffers' package names when working with external channel emmulator
- Address review comments: use pthread directly, update calloc_or_fail, simplify CMake

targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb.sa.band78.fr1.106PRB.usrpb210.conf

@@ -248,3 +248,15 @@ e2_agent = {
   #sm_dir = "/path/where/the/SMs/are/located/"
   sm_dir = "/usr/local/lib/flexric/"
 };
+
+vrtsim :
+{
+  #role = "server";                    # "server" or "client"
+  #client-num-rx-antennas = 1;           # publish expected client RX count
+  #cirdb = 1;                             # Enable CIR DB taps
+  # cirdb_yaml = "/path/to/vrtsim.yaml";
+  # cirdb_file = "/path/to/cir_db.bin";
+  cirdb_model_id = 0;                 # 0..4 for 3GPP TDL-A..E
+  cirdb_ds_ns = 10.0;                 # RMS delay spread in ns (10, 30)
+  cirdb_speed_mps = 1.5;              # UE speed in m/s (1.5, 30)
+};