docs: add browser-actions interaction-probe cookbook example

[chubes4]

Summary

Adds the reference cookbook example for the wordpress.browser-actions interaction probe shipped in #311. It boots WordPress trunk (so the editor stack runs on React 19), mounts a tiny vendor-neutral @wordpress/element demo plugin (no build step), and drives it with an ordered interaction script to prove the probe verifies a component works — not just that it renders.

This is the difference #311 was built for: a render-only probe ("the component mounts") would miss a regression where a component renders but its interaction is dead after a React major bump.

Scope note: an earlier revision of this PR shipped a plugin-specific recipe driving the data-machine-socials react-easy-crop modal. Per the platform's layer-purity convention (generic layers shouldn't hard-code vendor specifics), that worked consumer now lives in data-machine-socials' own repo, next to the code it guards. This PR keeps only the generic, swappable example.

What's added

• examples/recipes/cookbook/browser-actions-demo.json — the recipe (WP trunk, mounts the demo plugin, runs the seed, then a wordpress.browser-actions step).
• examples/recipes/cookbook/browser-actions-demo-plugin/browser-actions-demo.php — a tiny fixture plugin that renders a real @wordpress/element widget (click counter + bound range slider) into a Tools admin page. No build step.
• examples/recipes/cookbook/browser-actions-demo-seed.php — activates the plugin, logs in admin, emits the demo URL.
• examples/recipes/cookbook/README.md — documents the example and how to adapt it.

The interaction script (steps-json)

[
  { "kind": "navigate",   "url": "/wp-admin/tools.php?page=wp-codebox-browser-actions-demo", "waitFor": "load" },
  { "kind": "waitFor",    "selector": ".wpcb-increment" },
  { "kind": "screenshot", "name": "demo-loaded" },
  { "kind": "expect",     "selector": ".wpcb-count[data-count=\"0\"]", "state": "visible" },
  { "kind": "click",      "selector": ".wpcb-increment" },
  { "kind": "click",      "selector": ".wpcb-increment" },
  { "kind": "click",      "selector": ".wpcb-increment" },
  { "kind": "expect",     "selector": ".wpcb-count[data-count=\"3\"]", "state": "visible" },
  { "kind": "expect",     "selector": ".wpcb-threshold", "state": "visible" },
  { "kind": "evaluate",   "expression": "document.querySelector('.wpcb-count').getAttribute('data-count')", "assert": "3" },
  { "kind": "screenshot", "name": "after-increment" },
  { "kind": "fill",       "selector": ".wpcb-slider", "value": "80" },
  { "kind": "expect",     "selector": ".wpcb-slider-value[data-value=\"80\"]", "state": "visible" },
  { "kind": "evaluate",   "expression": "document.querySelector('.wpcb-slider-value').getAttribute('data-value')", "assert": "80" },
  { "kind": "screenshot", "name": "after-slider" }
]

Real run results (action-summary.json)

Ran end-to-end with npm run wp-codebox -- recipe-run --recipe ./examples/recipes/cookbook/browser-actions-demo.json --json on WordPress trunk / React 19:

success: true
steps: 15/15 ok
assertions: { "total": 6, "passed": 6, "failed": 0 }
errors: 0

#	kind	target	passed
1	expect	counter starts at 0	✅
2	expect	counter == 3 after 3 clicks	✅
3	expect	.wpcb-threshold message visible	✅
4	evaluate	data-count == "3"	✅
5	expect	slider output == 80	✅
6	evaluate	data-value == "80"	✅

Adapting it

Swap inputs.mounts[0].source to your plugin and rewrite steps-json to drive your own UI (click/fill/drag to interact, expect/evaluate to assert, screenshot to capture). Keep steps-json inline (not @file) if it contains an evaluate step so the recipe auto-grants the wordpress.browser-actions.evaluate policy capability.

Consumes the capability added in #311.

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Note: building the original socials-specific version surfaced a real latent bug in data-machine-socials (a double /wp-json/ prefix in apiFetch paths), filed as Extra-Chill/data-machine-socials#145.