perf: skip Who's Online payload when room state is unchanged

[i-am-chitti]

Description

Closes #268.

Server hashes the room's meaningful state; the client echoes its last hash. On a match the response carries last-seen timestamps instead of the payload, skipping cache_users(), per-user get_userdata() / get_avatar_url(), label building, and most of the JSON.

Review notes

• The unchanged reply carries a user_id → last-seen map, not a bare flag: the hash excludes date_gmt (it would flip every tick), so a flag alone breaks the idle dot for backgrounded tabs. Verified the dot still greys at 30s and clears at 60s, matching main.
• Display names and avatars aren't on the presence row, so profile changes won't reach other widgets until room state changes for another reason.

Testing

1. Log in as two users - admin in a normal window, an editor in an incognito window - both on the Dashboard. Position the windows side by side so neither covers the other; a fully covered window reports itself hidden and stops pinging, which invalidates the test.
2. Confirm the editor appears in the admin's Who's Online widget with a solid green dot.
3. In the admin window, open DevTools → Network and filter admin-ajax (presence rides WordPress's Heartbeat; it has no endpoint of its own). After the first tick, responses should carry presence-online-unchanged with a user_id → last-seen map instead of the full presence-online array, and drop noticeably in size.
4. In the editor window, switch to a second tab so the Dashboard is backgrounded. Leave it open - closing it deletes the presence entry and tests nothing.
5. Watch the editor's row in the admin window. The dot should turn grey at ~30s and the row should clear at ~60s - identical to main - while the admin's responses keep coming back as presence-online-unchanged.

Out of scope

heartbeat_received() runs on every admin screen but only the Dashboard consumes the data - off-Dashboard ticks still build an unread payload and can't be hash-suppressed. Will be raising a issue for this.

Measurement

To follow

Screenshots

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Implementation and tests

[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, joefusco.

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 cf08cff220bba40cd51e83ed0662b9cda8595f36. Auto-updates when you push.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.82%. Comparing base (8fdedf8) to head (cf08cff).
⚠️ Report is 39 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #288      +/-   ##
============================================
+ Coverage     75.87%   77.82%   +1.95%     
- Complexity      176      190      +14     
============================================
  Files            14       14              
  Lines          1832     1998     +166     
============================================
+ Hits           1390     1555     +165     
- Misses          442      443       +1     

Flag	Coverage Δ
multisite	77.82% <100.00%> (+1.95%)	⬆️
phpunit	76.62% <100.00%> (+2.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[josephfusco]

Looks good to me!