fix: lock I2C mutex while draining NotePing input

[zakoverflow]

Targets #239.

This addresses only the I2C mutex handling in NotePing():

• locks the I2C mutex around the pre-ping I2C drain path
• unlocks on normal and error exits from that drain path
• adds focused NotePing coverage that verifies I2C receive runs while the mutex is held and serial ping does not take the I2C mutex

Architecture docs: not updated; this is a local mutex-scope fix and does not change module boundaries, public APIs, or transport contracts.

Tests:

• docker CI image: ./scripts/run_unit_tests.sh (166/166 passed)