feat: add an Online view and column to the Network Users list

[josephfusco]

Part of the network presence stack, merged bottom-up:

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
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 (this one)
6. feat: add a Who's Online widget to the Network Admin dashboard #337 network dashboard widget

---

The summary is keyed by site and this screen is keyed by user.

Part of #298
Closes #322

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5, Claude Opus 5
Used for: Assisting with design, implementation, and testing

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[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 8cee7a26266fac8e99a2f2d17612dbb20dfd1482. Auto-updates when you push.}

[josephfusco]

#322 is closed by this one; test_the_network_online_view_is_not_narrowed_to_the_current_site asserts through a real WP_User_Query because the collision only shows up downstream of the filter.

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.73%. Comparing base (74ad3e1) to head (8cee7a2).

Additional details and impacted files

@@                         Coverage Diff                         @@
##             feature/network-sites-column     #336       +/-   ##
===================================================================
+ Coverage                           84.55%   96.73%   +12.18%     
  Complexity                            188      188               
===================================================================
  Files                                  16       17        +1     
  Lines                                2356     2418       +62     
===================================================================
+ Hits                                 1992     2339      +347     
+ Misses                                364       79      -285     

Flag	Coverage Δ
multisite	96.73% <90.47%> (?)
phpunit	82.38% <1.58%> (-2.17%)	⬇️

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