Add missing metadata fields

[mxtommy]

First time using Claude :-)

Implement #848

Backward Compatibility

• Existing constructor signature is unchanged
• add_entry emits identical JSON to before when no new fields are set (all new blocks are conditional)
• New members are value types that copy correctly with the default copy constructor

signalk_metadata.h

• SKAlarmState enum class with 6 values: kNominal, kNormal, kAlert, kWarn, kAlarm, kEmergency
• SKZone struct with optional lower_/upper_ bounds (NAN = not set), message_, and state_
• 5 new public members on SKMetadata: example_, supports_put_, display_scale_lower_, display_scale_upper_, zones_
  Private alarm_state_to_string() helper

signalk_metadata.cpp

• Constructor initializes sentinels: supports_put_ = -1, both scale bounds = NAN
• add_entry() conditionally serializes all 4 new fields
• alarm_state_to_string() maps enum values to the spec's lowercase strings

Usage example:

SKMetadata* meta = new SKMetadata("rpm", "Engine RPM");
meta->display_scale_lower_ = 0.0f;
meta->display_scale_upper_ = 8000.0f;
meta->zones_.push_back(SKZone(SKAlarmState::kNominal, "Normal range", 600.0f, 6500.0f));
meta->zones_.push_back(SKZone(SKAlarmState::kAlarm, "Overspeed", 7000.0f));
meta->supports_put_ = 0;  // false

[mairas]

Sorry about the delay! Please wait a bit - I'm having some discussion with the notification/alert future, related to this: https://github.com/hatlabs/signalk-alert-manager

[mxtommy]

Oh that's so cool :-)

No worries will be months until I'm in the water! I'm useing Waveshare ESP32-S3-Relay-6CH for some digital switching, and it needs supports_put :-D