Add Modbus RTU link to the EA3-T4CL touchscreen

[mllewellyn]

Summary

• Adds an hmi module that runs this board as a Modbus RTU slave on Serial1, wired through an external RS-232 transceiver to the EA3-T4CL's serial port (no P1000 base module involved).
• Mirrors the operator-facing status lines (mode, start-ignored reasons, cancel-cook results) to a status-line register block the display can bind a text tag to.
• Reads two momentary coils for start / cancel-cook from the screen, feeding into the exact same code paths as the physical buttons and typed serial commands.
• New dependencies: arduino-libraries/ArduinoModbus and arduino-libraries/ArduinoRS485 (added to the p1am_200 env only).

Register map (see include/Config.h)

• Holding registers 0..15: 32-char ASCII status line, 2 bytes/register.
• Coil 0: start (momentary write, self-clears on read).
• Coil 1: cancel cook (same).

The C-more Micro project needs a matching Modbus RTU master config at slave ID 1 / 19200 baud, a string tag over those registers, and two momentary-write buttons for the coils — that side is configured in AutomationDirect's EA-MG-PGMSW software, outside this repo.

Wiring

Serial1 (pins 13/14 on the P1AM-200's MKR header) through your RS-232 transceiver to the EA3-T4CL's RS-232 port (RJ12 or the DB15 RS-232/422/485 port set to RS-232 mode).

Test plan

• pio run -e p1am_200 builds clean
• pio test -e native still passes (16/16)
• Bench-verify against the real EA3-T4CL once the C-more project's tags are configured

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