Use modem-preset-relative SNR thresholds for signal quality

[jamesarich]

Problem

Our determineSignalQuality() in LoraSignalIndicator.kt uses fixed SNR/RSSI thresholds:

• SNR_GOOD_THRESHOLD = -7f
• SNR_FAIR_THRESHOLD = -15f
• RSSI_GOOD_THRESHOLD = -115
• RSSI_FAIR_THRESHOLD = -126

This means a node with SNR of -10 dB shows as "BAD" regardless of modem preset, even though on LongFast (SNR limit = -17.5 dB) that's actually 7.5 dB above the demodulation threshold — excellent signal.

Proposed Fix

Adopt modem-preset-relative thresholds (matching Meshtastic-Apple's approach):

Preset	SNR Limit	Good	Fair	Bad	None
LongFast	-17.5 dB	> limit	> limit - 5.5	≥ limit - 7.5	< limit - 7.5
LongSlow	-7.5 dB	> limit	> limit - 5.5	≥ limit - 7.5	< limit - 7.5
ShortFast	-7.5 dB	> limit	> limit - 5.5	≥ limit - 7.5	< limit - 7.5
MedSlow	-15.0 dB	> limit	> limit - 5.5	≥ limit - 7.5	< limit - 7.5

SNR limits per preset (from LoRa spreading factor physics):

• LongFast (SF11 BW250): -17.5
• LongModerate (SF11 BW125): -17.5
• LongSlow (SF12 BW125): -7.5
• LongTurbo (SF9 BW500): -12.5
• MedSlow (SF10 BW250): -15.0
• MedFast (SF9 BW250): -12.5
• ShortSlow (SF8 BW250): -10.0
• ShortFast (SF7 BW250): -7.5
• ShortTurbo (SF7 BW250): -7.5

Implementation Notes

• Requires plumbing the active modem preset from the connected device's LoRa config through to determineSignalQuality()
• The modem preset is available in the device's Config.LoRaConfig.modem_preset
• Cross-platform parity with Meshtastic-Apple (getSnrColor(snr:preset:) in LoRaSignalStrengthIndicator.swift)
• Affects: LoraSignalIndicator.kt, NodeItem.kt, NodeItemCompact.kt, BuildNodeDescription.kt

Ref: Apple implementation — https://github.com/meshtastic/Meshtastic-Apple/blob/main/Meshtastic/Views/Helpers/LoRaSignalStrengthIndicator.swift

[jamesarich]

@GUVWAF can you doublecheck this for me? Garth pointed me your way when I noticed this cross-platform discrepancy.

[GUVWAF]

The current approach of the Apple apps is slightly different than the proposed table here, as it also looks at RSSI:

https://github.com/meshtastic/Meshtastic-Apple/blob/01b7027bce722cefd4aaf0e23b89bddac21acdbd/Meshtastic/Views/Helpers/LoRaSignalStrengthIndicator.swift#L89-L95

In the end it's SNR that matters; the "packet RSSI" we use is the real strength of only the signal here, but to judge whether that's enough we would need to know the noise floor.
In case we want parity between the platforms I would say include the RSSI in the same way as the Apple apps do. But if we want to fix this altogether I would either remove the check for RSSI in both places.