[whatwg/dom] Add active template mutation records (PR #1518)

Add active template mutation records

Add an `activeTemplateAttributeFilter` member to `MutationObserverInit`, and `activeTemplateStatus` and `activeTemplateAttributes` attributes to `MutationRecord`. This lets observers identify the mutations that the HTML parser makes on behalf of a `template` element with a `for` attribute, including records with no added or removed nodes that mark the start and end of an update.

The insert, remove, and queue a tree mutation record algorithms gain optional activeTemplate and activeTemplateStatus parameters for HTML to pass.

`childList` no longer defaults to false, so that it can be implied when `activeTemplateAttributeFilter` is given.

Tests: https://github.com/web-platform-tests/wpt/pull/64641.

Helps with https://github.com/whatwg/html/issues/13008.

---

The HTML side is whatwg/html#13181. The two PRs need to land together.

I drafted this PR, the HTML PR, and the tests with an AI coding agent (Gemini in Antigravity), based on the proposed design in whatwg/html#13008 and the [explainer](https://github.com/WICG/declarative-partial-updates/blob/main/template-mutation-observer-explainer.md). I made the judgement calls: explicit optional parameters on insert, remove, and queue a tree mutation record; implying `childList` when the filter is given and throwing a `TypeError` when `childList` is explicitly false; and caching the attributes object per observer and per active template, so all records for one `<template for>` share an object that can be used as a `WeakMap` key.

- [ ] At least two implementers are interested (and none opposed):
   * …
   * …
- [x] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * https://github.com/web-platform-tests/wpt/pull/64641
- [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: …
   * Gecko: …
   * WebKit: …
- [ ] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: …
- [x] The top of this comment includes a [clear commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use.

(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/1518

-- Commit Summary --

  * Add active template mutation records

-- File Changes --

    M dom.bs (181)

-- Patch Links --

https://github.com/whatwg/dom/pull/1518.patch
https://github.com/whatwg/dom/pull/1518.diff

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