Fix cross-block text selection delete across nested blocks

[ellatrix]

What?

Closes #76233.

Backspace or Delete on a text selection that spans a nested list item (e.g. from the middle of an outer item to the middle of its own nested item) now deletes the selected range and merges the remaining content, instead of silently doing nothing.

Why?

The selection observer promoted ancestor/descendant selections to a block-level multi-selection, and the store's merge/delete paths bailed on any selection that did not start and end in the same block list.

How?

1. use-selection-observer.js: detect when one selection endpoint is an ancestor of the other and dispatch a rich-text selectionChange instead of promoting to multiSelect.
2. __unstableIsSelectionMergeable: drop the same-parent gate; determine document order via parent-chain comparison.
3. __unstableDeleteSelection: support ancestor/descendant endpoints; the merged content always lands in the ancestor (also for forward Delete, which previously targeted the descendant).

Notes

• Rebased onto trunk as a single commit (pre-rebase history preserved locally). Preserve nested list when deleting a selection across sibling list items #78776 landed the sibling-item half of this; the merged block now keeps the selection-end block's inner blocks, matching that PR's behavior, so the descendant's own children survive the merge.
• Known remaining gap (follow-up): unselected following siblings of the selection-end block inside intermediate containers are still dropped (e.g. in ab[ul[cd, xy]], selecting from mid-"ab" to mid-"cd" also removes "xy").
• A conservative alternative for the WP 7.1 cycle (fall back to selecting the ancestor block instead of partially merging) is proposed separately; this PR remains the full cross-level merge.

Testing Instructions

1. Create a list item "ab" with a nested item "cd".
2. Select from the middle of "ab" to the middle of "cd".
3. Press Backspace (or Delete).
4. Expect a single list item "ad" with the caret at the merge boundary.

Covered by three e2e tests in list.spec.js (2-level Backspace, 3-level Backspace, 2-level Delete).

Use of AI Tools

Authored with Claude Code under direction and review.

[github-actions]

Size Change: +176 B (0%)

Total Size: 7.73 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	422 kB	+176 B (+0.04%)

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