Merge remote-tracking branch 'origin/rrc-sibs' into integration_2026_w16 (!3983)

Add SIB 3,4 support, configurable SIB2 and refactor CU/DU SIB management

This MR makes neighbour and inter-frequency configuration drive how
SIB2/SIB3/SIB4 are built and sent from CU to DUs. It standardizes SIB
payloads as byte_array_t with typed SIB IDs across RRC, F1AP and MAC,
reducing ad‑hoc buffer handling. Neighbour parsing, validation and
lookup are tightened.

Changes

- Minor refactor to gNB neighbour parsing and storage (shared PLMN
  extraction, safer allocation, etc).
- Represent SIB containers uniformly as byte_array_t plus nr_sib_type_t,
  and adapt F1AP, MAC and RRC users to the new container API.
- Make SIB2 cell-reselection information fully config-driven with
  explicit bounds checking and SIB2 ASN.1 building from that config.
- Generate SIB3 intra-frequency neighbours from the per-cell neighbour
  list and propagate them from CU to DU over F1, with MAC
  decoding/attaching them to SystemInformation.
- Generate SIB4 inter-frequency neighbours from a new frequency_list
  plus neighbour SIB3/SIB4 offsets, and propagate them from CU to DU
  over F1, with MAC decoding/attaching them.
- Add basic ASN.1 round-trip tests for SIB2/SIB3/SIB4 and SIB4 range
  checks, and update RRC docs to describe the neighbour/inter-frequency
  configuration model.

Testing:

1. in gNB conf file:

    cu_sibs = (2, 3, 4);

2. Update neighbour config file with SIB3/SIB4 conf:

    (see documentation)

3. run gNB and UE as usual

---

Logs & configs: see MR on Gitlab

common/utils/nr/nr_common.h

@@ -13,6 +13,7 @@
 #include <stdlib.h>
 #include "assertions.h"
 #include "common/utils/utils.h"
+#include "common/utils/ds/byte_array.h"
 
 #define MAX_SI_GROUPS 3
 #define NR_MAX_PDSCH_TBS 3824
@@ -79,10 +80,32 @@ static inline const char *rnti_types(nr_rnti_type_t rr)
 // the total shift is 2 * 15, in dB scale thats 10log10(2^(15*2))
 #define SQ15_SQUARED_NORM_FACTOR_DB 90.3089986992
 
+typedef enum {
+  NR_SIB_1 = 1,
+  NR_SIB_2,
+  NR_SIB_3,
+  NR_SIB_4,
+  NR_SIB_5,
+  NR_SIB_6,
+  NR_SIB_7,
+  NR_SIB_8,
+  NR_SIB_9,
+  NR_SIB_10,
+  NR_SIB_11,
+  NR_SIB_12,
+  NR_SIB_13,
+  NR_SIB_14,
+  NR_SIB_15,
+  NR_SIB_16,
+  NR_SIB_17,
+  NR_SIB_18,
+  NR_SIB_19,
+  NR_SIB_20,
+  NR_SIB_21,
+} nr_sib_type_t;
+
 typedef struct {
-  uint8_t *SIB_buffer;
-  int SIB_size;
-  int SIB_type;
+  nr_sib_type_t SIB_type;
 } nr_SIBs_t;
 
 typedef struct nr_bandentry_s {