Initialize external RTC after power loss

[axhoff]

Summary

• detect lost-power state for discovered PCF8563 and DS3231 RTCs
• initialize an invalid external RTC from the already-initialized fallback clock
• preserve valid battery-backed RTC time
• add regression tests for both seed and preserve behavior

Root cause

AutoDiscoverRTCClock selected a PCF8563 after successful I2C initialization without checking its voltage-low/lost-power flag. On the XIAO expansion board without an RTC backup battery, a hard power loss could therefore leave untrustworthy clock contents (for example, a date in 2042). The companion's existing backward-time guard would then correctly reject the lower current time, leaving the node stuck on the invalid future timestamp.

Scope

• no protocol, packet, or storage changes
• existing backward-time guards remain unchanged
• valid external RTCs are unchanged
• DS3231 is included because it exposes the same lostPower() state and follows the same discovery path
• RV3028 and RX8130 behavior is unchanged

Validation

• pio test -e native — 15/15 tests passed
• pio run -e Xiao_S3_WIO_companion_radio_ble — passed
• git diff --check — passed

Fixes #2996