[maorun]

Summary

• Read the PCF85063A time before each NTP write. Calculate the drift in seconds and the rate in ppm since the previous sync.
• Store the last drift and rate in RTC memory. The state version is now 21. The old format gives a clean start.
• Show RTC DRIFT: +N S (+N PPM) in the diagnostics view. Show RTC DRIFT: -- before the second sync.
• Skip the rate for a sync less than 1 h after the previous one. Build the reference with the UTC offset of the previous sync, so a DST change shows no false hour.

Validation

• c++ -std=c++17 -Wall -Wextra -Werror tests/test_firmware_logic.cpp -o test_runner && ./test_runner: passed, 134 suites, local.
• arduino-cli compile rlcd_panel: passed, local.
• ./scripts/generate-display-preview.sh: passed, local.
• Review gate: 2 cycles. Cycle 1 had 3 MEDIUM and 5 LOW findings, all fixed. Cycle 2 had 4 LOW findings, all fixed.
• Not run: hardware test on the board.

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