What?

Introduces the ability to Limit Block Usage Within Parent, allowing a block (e.g., a "Login" button) to appear only once within the parent block.
Closes #38958

Why?

WordPress currently supports limiting a block’s usage to once per post/page via supports: { multiple: false }. But this limitation is too broad for nuanced layout structures. This enhancement allows greater design flexibility and cleaner content authoring by preventing UI redundancy within defined structural boundaries.

Example Use Case

• You have multiple Navigation blocks on a page (e.g., header and footer).
• You want to only allow one "Login" block per Navigation, not per page.
• With this feature, you can enforce that constraint per context, avoiding duplicate actions in a single section.

How?

• Add a new meta uniqueWithin which can be set to true or false.
• Prevent redundant insertion, dragging & duplication of block if it already exists in a parent.

Testing Instructions

1. Register a test block with uniqueWithin: true.
2. Add two Navigation blocks to a post/page.
3. Try inserting the limited block into both — it should work once per Navigation.
4. Try inserting a second one in the same Navigation.