Opportunistic clock sync from verified advert timestamps

[pelgraine]

Closes #1329. Related: #2132, #1332, #1882.

Problem

GPS-less repeaters lose their clock on every power cycle, reverting to the firmware default (~May 2024). This means their adverts carry stale timestamps until an admin manually syncs the clock via serial, the mobile app, or a remote management command. For inaccessible repeaters (rooftops, hilltops, solar-powered sites that reboot daily), manual sync is impractical.

Solution

Add an opt-in mechanism for nodes to sync their local RTC from signature-verified advert timestamps. When a node's clock is clearly wrong (before Jan 2025) and it receives a valid advert with a sane timestamp (Jan 2025 – Jan 2036), it adopts that timestamp.

The sync only fires when:

1. The sub-class opts in via shouldSyncClockFromMesh() (defaults to false)
2. The local clock is before Jan 2025 (clearly wrong / default state)
3. The incoming advert timestamp passes sanity checks
4. The advert signature has already been cryptographically verified
   Once the clock is set past Jan 2025, the sync stops firing — it won't overwrite a valid clock with subsequent adverts.

Changes

src/Mesh.h — New virtual method:

virtual bool shouldSyncClockFromMesh() const { return false; }

src/Mesh.cpp — 14 lines added after advert signature verification (inside the PAYLOAD_TYPE_ADVERT handler), before onAdvertRecv() call. Checks the opt-in gate, validates both local and incoming timestamps, and calls setCurrentTime() if appropriate.

examples/simple_repeater/MyMesh.h — Single-line override:

bool shouldSyncClockFromMesh() const override { return true; }

What this does NOT do

• Does not affect companion nodes, clients, or room servers (they don't override the method)
• Does not overwrite a valid clock — only fires when local time is clearly wrong
• Does not sync backward — a node with a stale clock can't drag others back
• Does not add any new preferences, CLI commands, or protocol changes
• Does not increase packet size or airtime

Testing

1. Flash a repeater without GPS/RTC
2. Confirm clock shows default (~May 2024) on boot
3. Wait for a valid advert from any peer with correct time
4. Confirm serial output shows clock synced from advert: <epoch>
5. Confirm subsequent adverts do NOT trigger further syncs
6. Confirm a repeater WITH GPS/RTC is unaffected (clock already valid)

Tested

• Station G2 repeater (both ESPNow bridge repeater and standard) - example screenshots from test:

• Heltec v3 espnow bridge repeater
• T-Echo Lite repeater (no gps) - example screenshots from test: