Add positionAreaContainingBlock option

[jpzwarte]

Closes #411

Adds a positionAreaContainingBlock option (default: true) to control how position-area is polyfilled.

• true (default): The target is wrapped with a <polyfill-position-area> element that approximates the containing block created by position-area. Alignment is applied via justify-self/align-self on the target within that wrapper.
• false: No wrapper element is inserted. Instead, the polyfill resolves the position-area alignment to concrete inset values and applies them directly on the target. This avoids breaking selectors that depend on a direct DOM relationship (e.g. > target, + target, :nth-child()), but comes with trade-offs around inset overrides, self-alignment, and overflow alignment (documented in the README and position-area.html).

[netlify]

✅ Deploy Preview for anchor-polyfill ready!

Name	Link
🔨 Latest commit	4b53b9f
🔍 Latest deploy log	https://app.netlify.com/projects/anchor-polyfill/deploys/6a2bd6b5dfba16000821370a
😎 Deploy Preview	https://deploy-preview-412--anchor-polyfill.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[netlify]

✅ Deploy Preview for anchor-position-wpt canceled.

Name	Link
🔨 Latest commit	4b53b9f
🔍 Latest deploy log	https://app.netlify.com/projects/anchor-position-wpt/deploys/6a2bd6b44c6ceb0008d5d07a

[jamesnw]

Thanks for taking a look at this! I haven't done a full review, but wanted to note the questions I will be considering when reviewing.

1. What are the differences between having a containing block and not? (See https://www.oddbird.net/2025/02/25/anchor-position-area/#a-new-containing-block)
2. Can we programmatically discover when a containing block will have an impact?
3. How do we want to expose this? Is a global setting for the polyfill sufficient? Or maybe some way of disabling the containing block for certain elements? A data-anchor-position-polyfill-disable-cb attribute, or --anchor-position-polyfill-disable custom property? I don't like the idea of leaving cleanup for when the polyfill can be removed, but control could be useful. I also want to avoid something that people opt out of because it fixes one problem, and then get surprised by the other differences.