Cover: declare the inner blocks template in block type settings

[ellatrix]

What?

Declares the Cover block's inner blocks template in its block type settings, like every other block in #80027, and teaches the placeholder to show whenever the block has no background and no content.

Alternative to #80028, following #80028 (comment). Built on top of #80027.

Why?

On trunk the placeholder worked by accident: the placeholder branch returns early without rendering the inner blocks container, so the template only applied once a background was chosen. With templates applied at insertion (#80027), the scaffolded paragraph would suppress the placeholder forever. Instead of scaffolding imperatively in every placeholder-exit handler (#80028), the placeholder condition absorbs the logic: a single empty scaffold paragraph does not count as content.

How?

1. Static template in settings. The fontSize: 'large' attribute is now unconditional; the theme font size check cannot be expressed statically.
2. hasInnerBlocks becomes hasInnerContent in the placeholder condition: more than one inner block, or one block that is not an empty paragraph.
3. The placeholder-exit handlers select the scaffolded paragraph so typing the title works immediately, preserving the trunk behavior of templateInsertUpdatesSelection. Forgetting a future handler only costs focus, never content.

Behavior changes:

• An inserted then abandoned Cover saves with an empty inner paragraph, consistent with List, Quote and Buttons.
• Removing the last background from a contentless Cover returns it to the placeholder (previously the invisible empty paragraph prevented this).

The image transform e2e test raced the upload: transforming before completion silently loses the image on trunk too, but the paragraph masked the placeholder so the test passed. It now waits for the final URL.

Testing Instructions

1. Insert a Cover block; the media/color placeholder appears as before.
2. Pick an overlay color or upload media; the title paragraph appears focused, type into it.
3. Remove the overlay color again without typing; the placeholder returns.
4. Transform a group with content but no background into a Cover; the content shows, no placeholder.

Testing Instructions for Keyboard

Insert a Cover, Tab to a color swatch in the placeholder, press Enter, and type the title.

Use of AI Tools

🤖 Generated with Claude Code

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Size Change: +84 B (0%)

Total Size: 7.67 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-library/index.min.js	333 kB	+84 B (+0.03%)

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