Normalize BMS -40C cell probe handling (ignore any 2)

[Copilot]

Summary

• Implements the -40°C cell-probe rule as: ignore up to any 2 probes as disconnected, and treat additional -40°C probes as real temperatures.
• Keeps existing low-temp alert channels (BMS_T1..T4) and adds no new alert types.

What Changed

• Removed probe-index-specific handling and removed the optional sanitizer override.
• Added shared 4-probe sanitization (sanitizeCellProbeTemps) that evaluates all probes together.
• Policy is deterministic and simple: first two -40°C readings in T1->T4 order are treated as disconnected (NaN); 3rd/4th remain valid -40°C.
• Updated BMS ingestion and probe transition logging to use the same sanitized values.
• Updated LVGL to trust sanitized telemetry values (no extra re-sanitization pass).

Resulting Behavior

• 0-2 probes at -40°C: those probes are ignored as disconnected.
• 3 probes at -40°C: one -40°C is kept as valid and can trigger low-temp alerts.
• 4 probes at -40°C: two -40°C values are kept as valid and can trigger low-temp alerts.

Tests

• platformio test -e native-test -f test_simplemonitor
• Result: 19/19 passing.
• Added coverage for:
  • 0/1/2/3/4 -40°C probe combinations
  • mixed -39°C + -40°C
  • < -40°C invalid handling
  • transition behavior (2 -> 3 -> 2 disconnected sentinels)