GPS: cache model and baudrate and skip full sweep every startup

[HarukiToreda]

Description

Boot was spending too much time sweeping GPS models and baud rates every time, making nodes slugish and unresponsive on first boot.
This change keeps startup fast by remembering the last known-good GPS settings, while still verifying the cached value is real on each boot.

What changed

New GPS cache flow

• Save detected GPS model + baud to /prefs/gps_probe_cache.dat.
• On next boot, load that cache first.
• Verify the cached model/baud is actually alive before trusting it.
• If cached settings fail verification:
  • clear cache as stale
  • mark GPS as not present for this boot
  • skip full sweep this boot
  • allow full probe again on next boot

Setup/runtime behavior updates

• GPS::setup() now:
  • tries cached probe first (once per boot)
  • falls back to normal probe only when no usable cache
  • saves cache after successful detection
• GPS::runOnce() now:
  • disables GPS for current boot if cached check failed / no detected model

Maintainability

• Added short comments around the new cache/probe blocks so the startup decision path is easier to follow.

How to test

1. Flash firmware and boot once with GPS connected.
2. Confirm log shows cache save/load messages.
3. Reboot and confirm it uses cached model/baud (no full probe sweep).
4. Boot with GPS disconnected:
   • should mark cached probe stale
   • should disable GPS for that boot
5. Reconnect GPS and reboot:
   • should run full probe again and cache fresh values.

GNSS Chips to test:

• L76K
• AG3335
• AGTM3332D
• Ublox 10
• Ublox 9
• Ublox 7
• Ublox 6
• UC6580

[chatgpt-codex-connector]

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[Copilot]

Pull request overview

This PR improves GPS startup responsiveness by persisting the last known-good GNSS model + baud rate to the filesystem and attempting a fast “cached verify” path on subsequent boots, avoiding a full model/baud sweep on every startup.

Changes:

• Add a versioned /prefs/gps_probe_cache.dat record to cache detected GNSS model and baud.
• On boot, try cached model/baud first and actively/passively verify the GPS is present before trusting it.
• If cached verification fails, clear the cache and suppress full probing for the remainder of that boot (GPS marked not present until reboot).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/gps/GPS.h	Adds cache/probe helper declarations and new state to track cached model/baud and boot-latched outcomes.
src/gps/GPS.cpp	Implements cache load/save/clear, cached presence verification logic, and integrates the cached flow into GPS::setup() / GPS::runOnce().

[fifieldt]

Good idea.

Just to note: changes to detection code of this extent should be tested on each GPS chip we support prior to merge.

Testing hitlist:

• L76K
• AG3335
• AGTM3332D
• Ublox 10
• Ublox 9
• Ublox 7
• Ublox 6
• UC6580

[HarukiToreda]

So far tested on
L76K
Ublox 9
Ublox 7