Upgrade GatherPress from 0.34.1 to 0.35.0

[bor0]

We're pinned to GatherPress 0.34.1 network-wide (gatherpress/gatherpress.php in wcorg-network-plugin-control.php). 0.35.0 is currently in beta (0.35.0-beta.1, tagged 2026-07-30) with its milestone due 2026-08-10 and all 44 tracked issues already closed, so it's close to a stable release.

Goal: stay on the latest GatherPress and pick up its fixes, while making sure our Groups/GatherPress integration (mu-plugins/wporg-groups-frontend, mu-plugins/groups/gatherpress-groups-tweaks.php, groups-site theme) doesn't regress.

Notable changes in 0.35.0 (from the beta.1 changelog)

• Minimum PHP raised to 8.1 (from 7.4) — not a blocker, we run PHP 8.3.
• Minimum WordPress raised to 7.0 — confirm our network is on 7.0+ before/at upgrade time.
• RSVP system rebuilt on a provider-registry architecture; new gatherpress_loaded action.
• RSVP settings vocabulary changed (enabled/disabled instead of on/off), settings page slugs gain a _settings suffix, values migrated automatically via the GatherPress Alpha migration routine.
• gatherpress_geocode_street_line filter removed, replaced by gatherpress_formatted_address — we don't currently use either, confirmed via grep.
• GatherPress Icon block removed in favor of core core/icon; Venue Map block dimensions/width handling reworked (Default Width setting removed).
• Venue map now mounted via the Interactivity API.

None of these directly touch code we grepped for in this repo, but the RSVP/settings rework and venue map changes are exactly the kind of thing that can silently break our custom REST endpoints and blocks — worth the full compat pass below rather than assuming it's fine.

Plan

• Confirm 0.35.0 has gone stable on WordPress.org (not just beta) before rolling out network-wide.
• Confirm network sites are on WP 7.0+.
• Update the pinned/tested version where applicable and deploy to a staging/testing group site first.
• Run PHPUnit (docker compose -f docker-compose.phpunit.yml up), covering the WordCamp Groups suite.
• Run E2E (npx playwright test / e2e-tests.yml workflow).
• Run the full manual groups-gatherpress-compat-test checklist (.claude/skills/groups-gatherpress-compat-test/SKILL.md): per-role browser pass, direct REST pass, GatherPress-off guard pass, security/perf sanity checks.
• Specifically re-verify RSVP flows and venue map/venue creation given the RSVP and Venue Map rework in this release.
• Roll out network-wide once all of the above pass clean.

Acceptance criteria

• GatherPress is upgraded from 0.34.1 to 0.35.0 network-wide.
• Automated (PHPUnit + E2E) and manual compat-checklist passes are all green, with results noted on this issue.
• No regressions in event creation/editing, RSVP, venue creation/map rendering, or member management across Organiser/Event Organiser/Member roles.