refactor(spi_flash): reorganize header files and improve API encapsulation

This commit reorganizes SPI flash header files to better reflect their
visibility and intended usage:

1. Rename `esp_flash_port/` to `esp_flash_chips/`:
   - Better reflects that these headers are for chip driver implementations
   - All chip driver headers moved to `esp_flash_chips/` directory
   - Added README.md explaining semi-public nature of these headers

2. Move internal headers to `esp_private/`:
   - `esp_flash_internal.h` -> `esp_private/esp_flash_internal.h`
   - `memspi_host_driver.h` -> `esp_private/memspi_host_driver.h`

3. Move chip driver related headers to `esp_flash_chips/`:
   - `esp_private/esp_flash_types.h` -> `esp_flash_chips/esp_flash_types.h`
   - `spi_flash/spi_flash_defs.h` -> `esp_flash_chips/spi_flash_defs.h`
   - `spi_flash_override.h` -> `esp_flash_chips/spi_flash_override.h`
   - All `spi_flash_chip_*.h` headers moved to `esp_flash_chips/`

4. Code improvements:
   - Remove unused includes (e.g., `spi_flash_override.h` from `cache_utils.c`)
   - Use public API `esp_flash_get_size()` instead of direct member access
   - Add `esp_flash_is_quad_mode` to linker.lf for IRAM placement

5. Documentation updates:
   - Add README.md in `esp_flash_chips/` explaining semi-public headers
   - Update programming guide with warnings about internal headers
   - Update both English and Chinese documentation

6. Update all references across the codebase:
   - Update includes in `spi_flash` component
   - Update `bootloader_support`, `app_update`, `esp_tee`, `espcoredump`
   - Update example projects

Breaking changes:
- Headers moved to new locations require include path updates
- `custom_flash_driver` example temporarily disabled until external
  components are updated

docs/en/migration-guides/release-6.x/6.0/peripherals.rst

@@ -310,9 +310,28 @@ SPI Flash Driver
 - New argument ``flags`` is added to ``esp_flash_os_functions_t::start``. Caller and implementer should handle this argument properly.
 - Kconfig option ``CONFIG_SPI_FLASH_ROM_DRIVER_PATCH`` has been removed. Considering that this option is unlikely to be widely used by users and may cause serious issues if misused, it has been decided to remove it.
 
-.. note::
+Header File Reorganization
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-    Note that enabling :ref:`CONFIG_FREERTOS_IN_IRAM` will increase IRAM usage. Consider this trade-off when optimizing for SPI performance.
+Several internal header files have been reorganized to better reflect their visibility and intended usage:
+
+- **Flash chip driver related headers** have been moved to ``esp_flash_chips/`` directory:
+  - ``spi_flash_chip_driver.h``
+  - ``spi_flash_chip_*.h``
+  - ``spi_flash_defs.h``
+  - ``spi_flash_override.h``
+  - ``esp_flash_types.h``
+  - The ``esp_flash_t`` structure definition has been moved from ``esp_flash.h`` to ``esp_flash_chips/esp_flash_types.h``. Applications should not access structure members directly; use the public APIs instead (e.g., use :cpp:func:`esp_flash_get_size` instead of accessing ``chip->size`` directly).
+  - The ``esp_flash_os_functions_t`` structure definition has been moved from ``esp_flash.h`` to ``esp_flash_chips/esp_flash_types.h``.
+  - The ``spi_flash_chip_t`` type forward declaration has been removed from ``esp_flash.h`` and all ROM headers (``components/esp_rom/esp32xx/include/esp32xx/rom/esp_flash.h``). The type is now only defined in ``esp_flash_chips/esp_flash_types.h``. Applications should not use this type directly; it is only intended for custom chip driver implementations.
+
+  .. note::
+
+      The headers in ``esp_flash_chips/`` are **semi-public** - they are intended for expert users who need to implement custom chip drivers for unsupported flash chips, but they are **not considered stable API** and may change without notice. For most use cases, you should use the public APIs in ``esp_flash.h`` instead. See :doc:`Override Driver for SPI Flash <../../../api-reference/peripherals/spi_flash/spi_flash_override_driver>` for more details.
+
+- **Internal headers** have been moved to ``esp_private/`` directory and not included in the public (stable) header files:
+  - ``esp_flash_internal.h``
+  - ``memspi_host_driver.h``
 
 Touch Element
 -------------