fix(core): preventDefault on Enter at start of ATX heading

[Jocs]

Summary

• Pressing Enter before the leading # in an ATX heading inserts a new empty paragraph above the heading, but atxHeadingContent.enterHandler forgot to call event.preventDefault(). The browser's native Enter behavior then runs on the contenteditable, splitting/cloning the heading's <span class="mu-content"> into orphan nodes without BLOCK_DOM_PROPERTY. A subsequent click resolves to such an orphan and Selection.getSelection crashes at anchorBlock.path. (setextHeadingContent already calls preventDefault() + stopPropagation() in the same branch — this aligns ATX with that.)
• Adds a defensive null-guard in Selection.getSelection so an unlinked DOM node returns null (the existing "no selection" outcome) instead of crashing if any other future path produces the same condition.
• Adds a unit test that drives AtxHeadingContent.prototype.enterHandler with a structurally-typed fake this and asserts preventDefault is called for both offset 0 and offset 1 in an H1.

Reproduction (pre-fix)

1. In the playground, place the caret before # in # 1. Headings.
2. Press Enter.
3. Click on the previous paragraph.
4. Uncaught TypeError: Cannot read properties of undefined (reading 'path') at Selection.getSelection.

Test plan

• pnpm --filter @muyajs/core test — 388/388 pass (new spec included)
• pnpm lint:types — clean
• Manual: run pnpm dev, reproduce the steps above, confirm no crash and the new empty paragraph is selectable

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes a crash triggered by pressing Enter at the start of an ATX heading by (1) preventing the browser’s native Enter behavior in AtxHeadingContent.enterHandler and (2) making Selection.getSelection() resilient to DOM nodes that aren’t linked to a Muya block.

Changes:

• Call event.preventDefault() + event.stopPropagation() when Enter inserts a paragraph above an ATX heading.
• Add a null-guard in Selection.getSelection() when BLOCK_DOM_PROPERTY is missing (return null instead of throwing).
• Add a unit test asserting preventDefault is called for offsets 0 and 1 in an H1 ATX heading.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/core/src/selection/index.ts	Adds a defensive guard to avoid crashing when selection resolves to an unlinked DOM node.
packages/core/src/block/content/atxHeadingContent/index.ts	Prevents native Enter behavior in the “insert paragraph above heading” branch (aligns with setext behavior).
packages/core/src/block/content/atxHeadingContent/tests/enterHandler.spec.ts	Adds regression coverage ensuring preventDefault is called for Enter at the start of an ATX heading.

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