Declare `coordinates` in the fieldset

[edalzell]

The addon reads and documents coordinates but never declares it, so GEO: only works if a site happens to define a matching field by hand. Part of giving #190 a complete, declared location contract instead of a set of conventions.

Field

resources/fieldsets/event.yaml, in the Location section:

• coordinates — type: group containing latitude and longitude, both type: float

Statamic 6 core has no map or coordinates fieldtype, so a group is the shape. It also matches what the addon already reads and what existing sites have on disk, so there's nothing to migrate.

Tests

TDD — write these failing first.

• a declared coordinates emits GEO: with both values
• validation rejects a non-numeric latitude or longitude
• partial coordinates still don't fatal and emit no GEO: — confirms the Multi-day ICS downloads drop location, URL, description and coordinates #192 guard holds once the field is declared and therefore augmented
• all four download routes

Docs

• move coordinates out of the documented-but-undeclared list and into the fieldset section
• document the latitude / longitude shape and the GEO: mapping in the field table from Reconcile the contradicting ICS field docs #191
• note it's optional, and only meaningful alongside a physical location

Branch

feature/coordinates-field off main. Needs #192; otherwise independent — can land before or after the online_url and location work.

Part of #190.