[BUGFIX] DocumentFragment support — {{#in-element}} with DocumentFragment targets

[Copilot]

Adds full DocumentFragment support to {{#in-element}}, including correct behaviour after the fragment's children have been moved into a real DOM container via appendChild().

Runtime Fixes (@glimmer/runtime)

• bounds.ts — clear(): Use firstNode().parentNode (the live current parent) instead of the stored parentElement(). After a DocumentFragment is appended to the DOM, the rendered nodes' actual parent is the container, not the now-empty fragment.
• element-builder.ts — NewTreeBuilder.resume(): Capture the live parent from firstNode().parentNode before resetting the block, so subsequent renders are inserted into the real container rather than the detached fragment.
• element-builder.ts — RemoteBlock destructor: Changed the guard from parentElement() === firstNode().parentNode (always false after fragment attachment, silently skipping cleanup) to firstNode().parentNode !== null (node is still attached to some parent), so content is correctly cleaned up on destroy in both the normal and DocumentFragment cases.

Test

Added a test (InElementDocumentFragmentSuite) that:

1. Renders reactive content into a detached DocumentFragment via {{#in-element}}.
2. Moves the fragment's children into a real DOM container with container.appendChild(fragment).
3. Calls rerender and verifies that text-node updates and new conditional elements both appear correctly in the container — using assert.step called from within the template and assert.verifySteps to document each reactive update.

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[github-actions]

📊 Package size report   0.03%↑

File	Before (Size / Brotli)	After (Size / Brotli)
dist/ember.debug.js	2.1 MB / 321.1 kB	0.05%↑2.1 MB / 0.04%↑321.3 kB
dist/ember.debug.js.map	3.4 MB / 635.1 kB	0.04%↑3.4 MB / 0.03%↑635.2 kB
dist/ember.prod.js	1.9 MB / 291.7 kB	0.06%↑1.9 MB / 0.07%↑291.9 kB
dist/ember.prod.js.map	3.3 MB / 607.8 kB	0.04%↑3.3 MB / 0.08%↑608.3 kB
dist/packages/shared-chunks/dynamic-DKOGdHsM.js	119.4 kB / 24.1 kB	0.3%↑119.7 kB / 0.3%↑24.2 kB
dist/packages/shared-chunks/element-builder-DRDkXgl3.js	11.7 kB / 2.6 kB	6%↑12.4 kB / 7%↑2.8 kB
Total (Includes all files)	17.1 MB / 3.2 MB	0.03%↑17.1 MB / 0.04%↑3.2 MB
Tarball size	3.9 MB	0.06%↑3.9 MB

_{🤖 This report was automatically generated by pkg-size-action}

[NullVoxPopuli]

I'm impressed that adding document-fragment support is such a small change it doesn't bust past our noise filter on the size comment (the map files do tho haha)

[NullVoxPopuli]

I made a GH workflow for more easily testing these out:
NullVoxPopuli/limber#2101

(including PR deploy)

[NullVoxPopuli]

Demo with some info: https://test-ember-source-copilot-ex.limber-glimdown.pages.dev/edit?c=JYWwDg9gTgLgBAYQuCA7Apq%2BAzKy4DkAAgOYA2oI6UA9AMbKQZYEDcAUKJLHAN5xo4AXzi58xdCABG1GiAgATYNmDU2nFD34woAQzoBrdAoDyUgFbo68EWJCEikmbSjp9MYADd09CGTJWHmgAzursDKjB8FG6MOhwALxwOvpGphaBABS87HBwDACuWABccAAMADS5cMCodKWZAJSJAHxwMegwAHSFWADUfVVCjRzs4SE4eiRUWIlwChB0BTPddK6x6AAii8uYMABiUytNo8GdACqg6BAFMJlNrXzVeRHBfuhdZBAkmbi603tGuwhBU4ABGMplEZjAA8cXAZA2LWqdAAFlYDMl0fkQu84GM8jDUVBkXldME4Lp2ugwLo9HE4AAFABKcAA7vEMMZkhA4CBdEZREc9hTXKgFNRdFIAtUYIpdABPUHYPwGClsm5kBRwLnaqnyJQqag8ylgMCYbUwbF-AGzOVY%2BKbEwAWVB1TZqOAaJxBS1%2BV0BTOcAA4hQQFQoCaSJ0cahsIHuVIFe1kNcMHB0GQg-zUP9aiQ4E7ncVqrxeDaVkIhAS%2BLwAMS1AC0mckeyF-0r1byeTiAA94LVKe3bTBnrWvgWAERNlsrOBSRQKydVsdlmJxHo3LArscwqS3OWoAR1CiGBJrmAbLq1OhVlo3mE0fcwQ%2Bk2s0GcBTvsR-wsCIuI2iAA&format=gjs

import Component from '@glimmer/component';
import { on } from '@ember/modifier';
import { trackedObject } from '@ember/reactive/collections';

const state = trackedObject({
  count: 0,
  inc: () => staet.count++,
});


const fragment = document.createDocumentFragment();

setTimeout(() => {
    console.log(fragment)
}, 100);

<template>
  check the console 

  <hr>
  as a separate PR we need to make fragments renderable
  today, folks would need a modifier to append the fragment to the DOM, 
  which could cause Glimmer to get confused by someone else managing DOM:
  {{fragment}}

  {{#in-element fragment}}
    text in a fragment
    {{log "in-element body"}}
    {{state.count}}

    <button onclick={{state.inc}}>inc</button>
  {{/in-element}}
</template>