fix: clear a user's presence when their account or site membership ends

[josephfusco]

Based on #333, off to the side of the network presence stack rather than in the line of it:

1. feat: add a network-wide presence summary table #332 summary table
2. feat: push each site's online set into the network summary #333 write path
   • fix: clear a user's presence when their account or site membership ends #342 user and site cleanup (this one)
3. feat: read the network summary as a capped snapshot #334 read path
4. feat: add an Online column to the Network Sites list #335 Sites list column
5. feat: add an Online view and column to the Network Users list #336 Users list
6. feat: add a Who's Online widget to the Network Admin dashboard #337 network dashboard widget

Merges any time after #333, independent of #334 and above.

---

@i-am-chitti wrote this in #339, which merged into feature/network-presence-screen after that branch was closed along with #313, so the commits are cherry-picked here with authorship intact. The only change is that the network test moved from tests/test-network-presence.php to tests/test-network-summary-push.php, since the restructure split that file into four.

Fixes #327

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Assisting with rebasing and testing

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props iamchitti.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

▶ Preview in WordPress Playground

Boots a fresh WordPress with this PR's presence-api build, seeds 5 demo users, and drops you on the dashboard.

_{Stress-test variant: 40 demo users · Built from 74d68f4486176332b784394c1e28472845419c4f. Auto-updates when you push.}