change "admin" into some custom "name" on installation

[F J Kaiser]

one of the most common things after you finished your installation is to add a new user with admin-permissions. then you log-out, log-in again with your new admin-username and delete the "admin"-user to make it harder to hack your blog.

i would like to suggest to let the user choose his username on installation, to avoid hacking (on blogs where the owner is not aware of this security issue) for the one side of blog owners and make life a little bit easier/installation and configuration a bit faster for the more advanced users.

[hakre]

Dupliacte of #10396

This builds on #11599 by giving awareness its own table (wp_presence) with a UNIQUE KEY (room, client_id). This resolves the problems the cache layer was solving at the source — one code path, one query per write, no race conditions.

## What changes

• Awareness writes become a single atomic INSERT ... ON DUPLICATE KEY UPDATE
• The collaboration table becomes a clean append-only update log
• Object cache branching and fallback logic no longer needed
• Net diff: -380 lines

## What ships (API only — no admin UI)

Minimal presence functions used internally by the collaboration server:

• wp_get_presence() / wp_set_presence() / wp_remove_presence()
• wp_remove_user_presence() / wp_delete_expired_presence_data()

The schema and API design are informed by the ​Presence API feature plugin, which explores UI features for a future release.

## Relationship to prior PRs

Forked from #11599 → #11256. Full commit history preserved. @peterwilsoncc's non-awareness improvements (test isolation, wp_recursive_ksort) are carried forward.

## Props

peterwilsoncc, jorbin, czarate, desrosj, zieladam, joefusco, paulkevan, mindctrl, mukesh27, dmonad, westonruter

## Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 4
Used for: Code review of #11599, architectural analysis comparing storage approaches, implementation of the presence table integration and test updates. All changes were reviewed and verified by me.

Applies the changes in #11599 to the 7.0 branch to create a patch for the ​https://github.com/WordPress/distributed-rtc-performance-testing/ repository.

## Use of AI Tools

None

Closing to focus on the single-table approach in #11256. The presence table concepts explored here can be revisited in a future release if needed.