Declare coordinates in the event fieldset

[edalzell]

Summary

• Declare optional coordinates (latitude / longitude floats) on the Location section of events::event
• Document the fieldset field and ICS GEO: mapping; note partial/non-numeric values are skipped safely
• Cover validation + all four ICS download routes

Non-breaking — targets 6.x.

Closes #194
Part of #190

Test plan

• vendor/bin/pest --filter=IcsControllerTest
• In CP, confirm Coordinates fields appear under Location on an event
• Save numeric lat/lng and confirm ICS includes GEO:
• Confirm non-numeric lat/lng is rejected by field validation

[edalzell]

Closing without merging.

Decision: ship nested location (group with place text + coordinates) only in 7.0, alongside the Prime cutover. Declaring top-level coordinates on 6.x would force a data migration for anyone who fills it and then upgrades to 7.

online_url on 6.x stays; Events continues to read undeclared top-level coordinates when present. Field declaration + nested shape moves to main after the next 6.x → main forward-merge.

Updates #194 / #190.

[edalzell]

Deferred to 7.0 nested location on main.