Merge remote-tracking branch 'origin/rrc-multi-cell' into integration_2026_w07 (!3783)

RRC: Multi-cell handling architecture

This MR implements a foundational multi-cell handling architecture for
RRC by separating DU management from cell management and establishing a
hierarchical cell organization within each DU container. The changes
enable proper multi-cell support where each DU can serve multiple cells,
addressing the previous limitation where DU and cell management were
tightly coupled with the assumption of one cell per DU.

Key Changes:

- Data Structure Separation: Separated for DU containers (DU-specific
  info) from cell containers (cell-specific data)
- Global DU and Cell Trees: Cells are now stored in global RB trees in
  RRC, similarly to DUs
- Du-specific cell seq_arr: within each DU container (du->cells) is
  stored a seq_arr of cells belonging to the DU
- UE Cell Association Framework: Introduced serving_cells array to track
  PCell and SCells for each UE with servCellIndex indexing
- Cell Management Library: Extracted cell management functions into
  rrc_cell_management.c/h for better testability and maintainability

Other relevant changes:

- F1AP Refactoring: cell list definitions, heap allocation to prevent
  stack overflow
- 5G-S-TMSI Helpers: Common helper functions for 3GPP TS 23.003
  compliant construction
- Integration Tests: Comprehensive test suite for DU lookup, cell
  lookup, and UE cell association
- Cell Duplication Checks: Validation in F1 DU configuration update to
  prevent duplicate cells
- Documentation: Updated RRC developer documentation with multi-cell
  architecture details

Validated with RFsim in re-establishment and handover scenarios.

doc/RRC/rrc-usage.md

@@ -41,15 +41,19 @@ For each DU, it prints:
 
 ```
 1 connected DUs
-[1] DU ID 3584 (gNB-OAI-DU) assoc_id 8: nrCellID 12345678, PCI 0, SSB ARFCN 641280
-    TDD: band 78 ARFCN 640008 SCS 30 (kHz) PRB 106
+[1] DU ID 3584 (gNB-OAI) integrated DU-CU: 1 cell
+  [1] nrCellID 12345678, PCI 0, Mode TDD, SSB ARFCN 641280
+      TDD: band 78 ARFCN 640008 SCS 30 (kHz) PRB 106
 ```
 
-i.e., an index (`[1]`), the DU ID and it's name, the SCTP association ID
-(`assoc_id 8`, cf. UE information), and DU specific information for the cell
-(cell ID, physical cell identity/PCI, the SSB frequency in ARFCN notation, the
-band and Point A ARFCN, subcarrier spacing/SCS, and the number of resource
-blocks/PRB). Only one cell per DU is supported.
+The output shows:
+- Total number of connected DUs
+- For each DU: an index (`[1]`), the DU ID and name, connection type (either `integrated DU-CU` for monolithic deployments or `assoc_id X` for split deployments), and the number of cells served by this DU
+- For each cell served by the DU: a cell index (`[1]`), the NR Cell ID, Physical Cell ID (PCI), operating mode (TDD or FDD), and SSB ARFCN
+- Cell-specific frequency information: for TDD mode, the band, Point A ARFCN, subcarrier spacing (SCS), and number of resource blocks (PRB); for FDD mode, separate DL and UL frequency information
+
+The RRC enables the support of multiple cells per DU (though currently each DU
+typically serves one cell in practice).
 
 As of now, it does not print information about connected CU-UPs or AMFs.