examples/calculator: Add an LVGL touchscreen calculator.

[aviralgarg05]

Summary

Add a small LVGL touchscreen calculator with the usual four operations,
division-by-zero handling, and cooperative shutdown.

The layout scales to the framebuffer. EXAMPLES_CALCULATOR_YOFFSET can
reserve rows for a supervisor bar without coupling the example to NXStore.

Impact

• New example, enabled through Kconfig
• Requires LVGL, a framebuffer, and a touchscreen
• Existing applications are unchanged

Testing

Host: macOS 26.5, arm64, xtensa-esp-elf-gcc 14.2.0.
Target: Waveshare ESP32-S3-Touch-LCD-7, 800x480 RGB565, GT911 touchscreen.

• nxstyle, tools/checkpatch.sh, codespell, and git diff --check
• built as a builtin and loadable module
• built with a 36-row supervisor offset
• checked arithmetic and division by zero on hardware
• started and closed it repeatedly with no task left behind

PR verification Self-Check

• This PR introduces one focused change.
• I have updated all required description fields above.
• I have reviewed and signed every commit.
• This PR adheres to the current contribution and coding guidelines.
• My PR is still work in progress.
• My PR is ready for review and can be safely merged.

[github-actions]

🔗 Cross-repo PR dependencies

The read-only Build run reported the following dependent PR(s) and fetched head SHA(s):

• system/nxstore: Add LVGL touchscreen app-store frontend. #3643 @ e93ea02

CI run: https://github.com/apache/nuttx-apps/actions/runs/31813509921

[github-actions]

🔗 Cross-repo PR dependencies

The read-only Build run reported the following dependent PR(s) and fetched head SHA(s):

• system/nxstore: Add LVGL touchscreen app-store frontend. #3643 @ 68f50e7

CI run: https://github.com/apache/nuttx-apps/actions/runs/31878524647

[github-actions]

🔗 Cross-repo PR dependencies

The read-only Build run reported the following dependent PR(s) and fetched head SHA(s):

• system/nxstore: Add LVGL touchscreen app-store frontend. #3643 @ 76204e7

CI run: https://github.com/apache/nuttx-apps/actions/runs/31879830797

[github-actions]

🔗 Cross-repo PR dependencies

The read-only Build run reported the following dependent PR(s) and fetched head SHA(s):

• system/nxstore: Add LVGL touchscreen app-store frontend. #3643 @ 76204e7

CI run: https://github.com/apache/nuttx-apps/actions/runs/31882801968

[github-actions]

🔗 Cross-repo PR dependencies

The read-only Build run reported the following dependent PR(s) and fetched head SHA(s):

• system/nxstore: Add LVGL touchscreen app-store frontend. #3643 @ 2c6b64d

CI run: https://github.com/apache/nuttx-apps/actions/runs/31955922933