RRC multi-cell: add data structure definitions for multi-cell architecture

Add foundational data structures in nr_rrc_defs.h for multi-cell handling
in RRC, separating DU management from cell management.
Design uses a global cell tree for O(log N) lookups while keeping cell
ownership in DUs for locality and maintenance.

The implementation maintains backward compatibility and follows the same
patterns as existing DU and CUUP management in the codebase, i.e. RB-trees
with logarithmic complexity for scalable multi-cell deployments.

- Cell container structures:
  * nr_rrc_cell_container_t: cell-specific data (cell_id, pci, plmn, tac,
    frequency info, mib, sib1, mtc)
  * nr_rrc_cell_info_t: TDD/FDD modes; frequency structs nr_rrc_freq_info_t,
    nr_rrc_tdd_info_t, nr_rrc_fdd_info_t

- UE serving cell tracking:
  * ue_serving_cell_t: nci, serving_cell_id (ServCellIndex TS 38.331,
    0=PCell, 1-31=SCell), assoc_id for fast DU lookup
  * gNB_RRC_UE_t: seq_arr_t serving_cells, with PCell always being the
    first in the sequence

- DU container refactoring:
  * nr_rrc_du_container_t: DU-only fields (gNB_DU_id, gNB_DU_name, rrc_ver);
    seq_arr_t cells for per-DU iteration; setup_req/mib/sib1/mtc kept for
    backward compatibility

- Global cell tree:
  * gNB_RRC_INST_s: RB_HEAD(rrc_cell_tree, nr_rrc_cell_container_t) cells,
    num_cells (data only; lookup helpers get_cell_by_cell_id/get_cell_by_pci
    to be added separately)

openair2/RRC/NR/nr_rrc_defs.h

@@ -166,6 +166,21 @@ typedef struct {
   pdusession_level_qos_parameter_t qos;
 } nr_rrc_qos_t;
 
+/** @brief UE serving cell information
+ * @note ServCellIndex is a short identity used to uniquely identify a serving cell
+ *       (PCell, PSCell, or SCell) across cell groups (TS 38.331).
+ *       Value 0 applies for the PCell, while the SCellIndex that has previously
+ *       been assigned applies for SCells.
+ * @note Range: 0..maxNrofServingCells-1 where maxNrofServingCells = 32 */
+typedef struct {
+  /* NR Cell Identity (cell_id) */
+  uint64_t nci;
+  /* ServCellIndex (TS 38.331): 0 = PCell, 1-31 = SCell */
+  uint8_t serving_cell_id;
+  /* SCTP association ID of the DU that owns this cell (for fast lookup) */
+  sctp_assoc_t assoc_id;
+} ue_serving_cell_t;
+
 /* forward declaration */
 typedef struct nr_handover_context_s nr_handover_context_t;
 
@@ -208,6 +223,9 @@ typedef struct gNB_RRC_UE_s {
   NR_EstablishmentCause_t            establishment_cause;
 
   uint64_t nr_cellid;
+  /* Dynamic array of UE serving cells */
+  seq_arr_t serving_cells; /* ue_serving_cell_t */
+
   uint32_t                           rrc_ue_id;
   uint64_t amf_ue_ngap_id;
   // Globally Unique AMF Identifier
@@ -335,15 +353,69 @@ typedef struct cucp_cuup_if_s {
   cucp_cuup_bearer_context_release_func_t bearer_context_release;
 } cucp_cuup_if_t;
 
+typedef struct {
+  int band;
+  uint32_t arfcn;
+  uint8_t scs;
+  uint16_t nrb;
+} nr_rrc_freq_info_t;
+
+typedef struct {
+  nr_rrc_freq_info_t dlul;
+} nr_rrc_tdd_info_t;
+
+typedef struct {
+  nr_rrc_freq_info_t dl;
+  nr_rrc_freq_info_t ul;
+} nr_rrc_fdd_info_t;
+
+typedef struct {
+  /* operating mode: TDD or FDD */
+  enum { NR_MODE_TDD = 0, NR_MODE_FDD = 1 } mode;
+  uint64_t cell_id;
+  uint16_t pci;
+  union {
+    nr_rrc_tdd_info_t tdd;
+    nr_rrc_fdd_info_t fdd;
+  };
+  plmn_id_t plmn;
+  uint16_t tac;
+} nr_rrc_cell_info_t;
+
+typedef struct nr_rrc_cell_container_t {
+  /* Tree-related data */
+  RB_ENTRY(nr_rrc_cell_container_t) entries;
+  /* transport association */
+  sctp_assoc_t assoc_id;
+  /* Cell-only RRC-local info */
+  nr_rrc_cell_info_t info;
+  /* MIB message (6.2.2 TS 38.331) */
+  NR_MIB_t *mib;
+  /* SIB1 message (6.2.2 TS 38.331) */
+  NR_SIB1_t *sib1;
+  /* MeasurementTimingConfiguration inter-node (TS 38.331) */
+  NR_MeasurementTimingConfiguration_t *mtc;
+} nr_rrc_cell_container_t;
+
 typedef struct nr_rrc_du_container_t {
   /* Tree-related data */
   RB_ENTRY(nr_rrc_du_container_t) entries;
-
+  /* DU-only information */
+  /* Transport association identifier for this DU */
   sctp_assoc_t assoc_id;
+  /* DU identity */
+  uint64_t gNB_DU_id;
+  /* DU name */
+  char *gNB_DU_name;
+  /* RRC version */
+  uint8_t rrc_ver[3];
+  // Cell-specific information (F1AP Setup Request, MIB, SIB1, MeasurementTimingConfiguration)
   f1ap_setup_req_t *setup_req;
   NR_MIB_t *mib;
   NR_SIB1_t *sib1;
   NR_MeasurementTimingConfiguration_t *mtc;
+  /* Cells, indexed by cell_id */
+  seq_arr_t cells; /* nr_rrc_cell_container_t* */
 } nr_rrc_du_container_t;
 
 typedef struct nr_rrc_cuup_container_t {
@@ -387,6 +459,10 @@ typedef struct gNB_RRC_INST_s {
   RB_HEAD(rrc_du_tree, nr_rrc_du_container_t) dus; // DUs, indexed by assoc_id
   size_t num_dus;
 
+  /* Global cell tree, indexed by cell_id */
+  RB_HEAD(rrc_cell_tree, nr_rrc_cell_container_t) cells;
+  size_t num_cells;
+
   RB_HEAD(rrc_cuup_tree, nr_rrc_cuup_container_t) cuups; // CU-UPs, indexed by assoc_id
   size_t num_cuups;