[codex] Add J-Calendar Z21 ESP32 profile

[eele83]

Summary

• add a J-Calendar ESP32 board profile with the observed GPIO pinout
• add a GDEQ042Z21 / GxEPD2_420c_Z21 tricolor display path
• add a 4MB OTA partition table and PlatformIO environment for this hardware
• add regression coverage for the profile, partition layout, and Z21 2bpp red/white mapping

Hardware verified

• ESP32-D0WD-V3 rev v3.1
• 4MB flash
• J-Calendar firmware 1.1.9 z21 panel match
• 4.2 inch 400x300 BWR panel using GxEPD2_420c_Z21

Validation

• python3 -m unittest firmware/tests/test_jcalendar_z21_profile.py
• pio run -e epd_42_jcalendar_z21_esp32dev
• flashed to the J-Calendar ESP32 device and verified the setup AP starts
• visually verified black/white/red output after fixing the Z21 2bpp red/white mapping

Notes

This intentionally adds a dedicated profile instead of runtime pin or panel detection. The Z21 panel uses a dedicated 2bpp mapping where black is 0x00, white is 0x01, and red accepts 0x02/0x03.