Collaboration: Replace post meta storage with dedicated database table

[josephfusco]

The real-time collaboration sync layer currently stores messages as post meta, which works but creates side effects at scale. This moves it to a single dedicated wp_collaboration table purpose-built for the workload.

Table Definition

CREATE TABLE wp_collaboration (
    id bigint(20) unsigned NOT NULL auto_increment,
    room varchar(191) NOT NULL default '',
    type varchar(32) NOT NULL default '',
    client_id varchar(32) NOT NULL default '',
    user_id bigint(20) unsigned NOT NULL default '0',
    update_value longtext NOT NULL,
    date_gmt datetime NOT NULL default '0000-00-00 00:00:00',
    PRIMARY KEY  (id),
    KEY room (room, id),
    KEY date_gmt (date_gmt)
);

Testing

npm run env:cli -- core update-db

Testers who already had the local env running before checking out your branch will hit the upgrade.php redirect without it. A fresh npm run env:install would also work, but core update-db is non-destructive and faster.

# Unit tests
npm run test:php -- --filter WP_Test_REST_Collaboration_Server

# E2E tests — 3 concurrent users (presence, sync, undo/redo)
npm run test:e2e -- tests/e2e/specs/collaboration/

Note for beta testers: If upgrading from a previous beta, the old wp_sync_updates table is no longer used and can be dropped: npm run env:cli -- db query "DROP TABLE IF EXISTS wp_sync_updates"

Use of AI Tools

Co-authored with Claude Code (Opus 4.6), used to synthesize discussion across related tickets and PRs into a single implementation. All code was reviewed and tested before submission.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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