Add `online_url` for Zoom and livestream links

[edalzell]

First half of #190: a dedicated field for a Zoom or livestream link, so it no longer has to be smuggled through location.

Purely additive. link and the Str::isUrl($location) sniff keep working, so no existing site's output changes — they come out in the follow-up that adds location. Safe to ship into Prime sites: Prime declares no online_url, so nothing collides.

Field

resources/fieldsets/event.yaml — new Location section, following the existing times_sections pattern:

• location_section — type: section, display "Location"
• online_url — type: text, input_type: url, validate: [nullable, url], localizable: false, display "Online URL", instructions naming Zoom / livestream

localizable: false because it's one shared meeting link across sites.

Resolution

src/Types/Event.php — extend existing eventUrl() resolving in order:

1. online_url
2. link (deprecated)
3. location, when it's a string that passes Str::isUrl() (deprecated)

Mark both fallbacks @deprecated, removed in 7.0.

Do not rename eventUrl() → icsUrl() on 6.x. Renaming a protected method is a breaking change for subclasses; keep the name here and only rename in 7.0 if desired (#195).

ICS

online_url → URL:. Deliberately no CONFERENCE and no X-GOOGLE-CONFERENCE: Google discards conference data on ICS import, and RFC 7986 is inert in every mainstream client. Clients that surface a join button (Fantastical and friends) pattern-match the provider URL out of the event content rather than reading a property. Reasoning in #190.

Tests

TDD — write these failing first.

• online_url emits URL:
• online_url wins over link when both are set
• deprecated link alone still emits URL:
• deprecated URL-valued location alone still emits both URL: and LOCATION: (unchanged behaviour)
• all four download routes: single-day, recurring, multi-day single-date, multi-day whole-event

Docs

• online_url added to the ICS field table from Reconcile the contradicting ICS field docs #191, with its URL: mapping and its precedence over link
• fieldset section covering the new field — optional, and combinable with a physical location for a hybrid event
• link marked deprecated, removed in 7.0

Branch

feature/online-url off 6.x (non-breaking / additive). Needs #192 for decorate(). Forward-merge to main with the rest of the 6.x stack.

Part of #190.

[edalzell]

Done in #209