Markers ignore thold state, so an alarming Device looks healthy

[somethingwithproof]

Is your feature request related to a problem? Please describe.

Markers are coloured from host.status alone, which is up, recovering, down, disabled or undefined.

A Device that is up but breaching a threshold looks identical to one that is perfectly healthy. On a map whose purpose is showing where trouble is, an alarming Device rendering green is the wrong answer.

Describe the solution you'd like

Surface thold state in the marker colour or badge when the plugin is installed, and fall back to the current behaviour when it is not.

There is precedent for the dependency: plugin_monitor declares requires = thold:1.2.1 in its INFO, so cross-plugin dependency is established practice in this org. gpsmap contains no reference to thold today.

The dependency should stay soft. gpsmap must keep working without thold, so this is a capability check rather than a hard requirement.

Describe alternatives you've considered

A separate overlay toggle rather than changing marker colour. Less disruptive to anyone reading the existing colours, but it hides the information behind another control.

Additional context

Depends on the diagnostics work in #97 landing first, so that an unexpected marker colour is explainable rather than mysterious.

[somethingwithproof]

Implementation is now in #111: when thold is enabled and its table is available, an otherwise-up Device with an active enabled threshold is emitted as alert and uses the configured Down icon. The join is aggregate and optional, so gpsmap remains standalone. No other open PR implements this behavior.