feat: read the network summary as a capped snapshot

[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 (this one)
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

---

The rows #333 writes have nothing reading them, and #310 asked whether that read holds up at large-network scale.

Part of #298
Closes #321
Closes #324

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]

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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 c323b1fbdc5373ae9ab77e2bca4e33614b66942f. Auto-updates when you push.}

[josephfusco]

Review findings from #299 and #313 that land here are #321 and #324, both closed by this one, with #327, #328, and #344 still open against the same code.

[codecov]

Codecov Report

❌ Patch coverage is 95.42857% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.91%. Comparing base (0b06e38) to head (129e066).

Additional details and impacted files

@@                        Coverage Diff                         @@
##             feature/network-summary-push     #334      +/-   ##
==================================================================
+ Coverage                           92.07%   96.91%   +4.83%     
  Complexity                            190      190              
==================================================================
  Files                                  15       15              
  Lines                                2159     2334     +175     
==================================================================
+ Hits                                 1988     2262     +274     
+ Misses                                171       72      -99     

Flag	Coverage Δ
multisite	96.91% <95.42%> (?)
phpunit	85.17% <0.00%> (-6.91%)	⬇️

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