[LiveComponent] Re-emit render hooks as live:render:* DOM events

[haraldpdl]

Q	A
Bug fix?	no
New feature?	yes
Deprecations?	no
Documentation?	yes
Issues	Related to #1038, #1676
License	MIT

Re-emits the existing render:started and render:finished JS hooks as bubbling DOM events on the component's root element, alongside the already-public live:connect / live:disconnect. Purely additive, no BC concern.

element.addEventListener('live:render:finished', (event) => {
    // event.detail.component, event.detail.controller
});

element.addEventListener('live:render:started', (event) => {
    // event.detail.html, event.detail.backendResponse, event.detail.controls
});

The motivation is the longstanding pain in #1038 / #1676 (both closed by stale-bot, neither resolved): third-party Stimulus controllers that need to react to a render have to import getComponent, await it inside initialize(), and register against the JS hook API, which is racey on first mount and only fires for re-renders. Falling back to a MutationObserver works but observes every DOM change rather than render boundaries. A bubbling DOM event lets any code subscribe with a single addEventListener, no async setup, no race.

Implementation re-uses the existing dispatchEvent helper that already powers live:connect / live:disconnect. Test in test/unit/controller/render.test.ts, doc section under "JavaScript Component Hooks > DOM Events", CHANGELOG entry under 3.1.

[github-actions]

📊 Packages dist files size difference

Thanks for the PR! Here is the difference in size of the packages dist files between the base branch and the PR.
Please review the changes and make sure they are expected.

File	Before (Size / Gzip)	After (Size / Gzip)
LiveComponent
live_controller.js	82.61 kB / 18.38 kB	82.88 kB0% / 18.42 kB0%