doc: migration guide: add Fast Pair partition overlay compatible migration

Added a Google Fast Pair section under "Libraries / Recommended
changes" of the v3.4 migration guide that documents the migration of
the Fast Pair sample partition overlays from the legacy
fixed-partitions compatible string to the new zephyr,mapped-partition
compatible string.

The new section is anchored as migration_3.4_google_fast_pair so that
other parts of the migration guide can reference it.

The migration is generic and applies to all Fast Pair samples and
board targets, but the section also highlights that it is mandatory on
nRF53 Series board targets: after the recent Zephyr SoC devicetree
updates that added a ranges translation to the cpunet flash node, the
legacy fixed-partitions style makes the linker double-count the flash
base address. Without the migration, the network core image is linked
outside the valid flash range and nrfutil rejects programming.

Ref: NCSDK-38010

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>

Author: kapi-no

doc/nrf/releases_and_maturity/migration/migration_guide_3.4.rst

@@ -146,6 +146,69 @@ Libraries
 
 This section describes the changes related to libraries.
 
+.. _migration_3.4_google_fast_pair:
+
+Google Fast Pair
+----------------
+
+.. toggle::
+
+   For applications and samples using the :ref:`bt_fast_pair_readme` library:
+
+   * The devicetree (DTS) partition overlays of the Fast Pair samples and Fast Pair-enabled board targets have been migrated from the legacy ``fixed-partitions`` compatible string to the new ``zephyr,mapped-partition`` compatible string introduced in Zephyr.
+     The new layout matches the output of the Partition Manager-to-DTS helper script (:file:`scripts/pm_to_dts.py`) and aligns the Fast Pair sample overlays with the partition binding convention adopted by the rest of the Zephyr SoC devicetree.
+
+     If your application uses a Fast Pair DTS partition overlay derived from earlier |NCS| releases, update each :file:`<board_target>.overlay` file as follows:
+
+     * On the ``partitions`` grouping node, replace the ``compatible = "fixed-partitions";`` property with an empty ``ranges;`` property.
+     * On every child partition node (for example, ``boot_partition``, ``slot0_partition``, ``slot1_partition``, ``bt_fast_pair_partition``, and ``storage_partition``), add the ``compatible = "zephyr,mapped-partition";`` property.
+
+     For example, replace the following overlay snippet:
+
+     .. code-block:: devicetree
+
+        partitions {
+            compatible = "fixed-partitions";
+            #address-cells = <1>;
+            #size-cells = <1>;
+
+            slot0_partition: partition@0 {
+                label = "image-0";
+                reg = <0x0 DT_SIZE_K(996)>;
+            };
+        };
+
+     With the following:
+
+     .. code-block:: devicetree
+
+        partitions {
+            ranges;
+            #address-cells = <1>;
+            #size-cells = <1>;
+
+            slot0_partition: partition@0 {
+                compatible = "zephyr,mapped-partition";
+                label = "image-0";
+                reg = <0x0 DT_SIZE_K(996)>;
+            };
+        };
+
+     Refer to the partition overlays under the :file:`nrf/samples/bluetooth/fast_pair/locator_tag/` and :file:`nrf/samples/bluetooth/fast_pair/input_device/` sample directories for complete examples of the updated layout.
+
+     .. important::
+        This migration is mandatory on nRF53 Series board targets.
+        Recent Zephyr updates added a ``ranges`` translation to the ``flash1`` SoC node (network core flash), so ``DT_REG_ADDR()`` on partition nodes now returns their bus address rather than their in-flash offset.
+        With the legacy ``fixed-partitions`` compatible, the Zephyr linker still uses ``CONFIG_FLASH_BASE_ADDRESS + CONFIG_FLASH_LOAD_OFFSET`` to place the network core code partition, which double-counts the ``flash1`` base.
+        As a result, the network core image is linked at an address outside the valid flash range and ``nrfutil`` rejects programming with the following message:
+
+        .. code-block:: console
+
+           Device error: Address range 0x02008800..0x020262ac is outside the memory ranges defined for programming through the Network core (Generic)
+
+        Switching to ``zephyr,mapped-partition`` makes the linker derive the network core code partition's load address directly from ``DT_REG_ADDR(zephyr_code_partition)``, which yields the correct bus address and avoids the double-counting.
+        On the application core, where ``flash0`` uses identity ``ranges``, both compatibles produce the same addresses, but updating the overlays is still recommended for consistency.
+
 nRF Cloud library
 -----------------