fix: respect nonconsecutive heading levels in markdown shortcuts

[huyanxius]

Summary

Respect each explicitly allowed heading level in Markdown shortcuts. With allowedHeadingLevels: [2, 4], typing ### currently creates an h3 because the transformer matches the entire min/max range.

• Build the heading matcher from the configured levels and preserve the captured marker used to choose the heading tag.
• Skip the heading transformer for an empty allowlist.
• Add root, nested-editor, and table-cell registration coverage, plus real Lexical shortcut-transform tests.
• Add a minimal Ladle example and Playwright typing cases for all six heading levels.

This addresses the remaining nonconsecutive-level behavior after #952.

Regression evidence

On unchanged main (122347c), the Lexical shortcut test with [2, 4] fails after inserting a space after ###: the resulting node is heading, but should remain paragraph. The other five heading-level cases pass. All six pass with this change.

The focused suite also checks default, contiguous, nonconsecutive, unsorted, repeated, single-level, and empty configurations, including the marker capture used for the resulting heading level.

Verification

Passed locally:

• npm run lint
• npm run typecheck
• npm run test:once: 154 passed, 1 skipped, 1 todo
• Focused shortcut suite: 33 passed
• npm run test:lexical-versions: 24 packages at 0.48.0
• npm run build
• npm run build:docs:api
• npm exec ladle -- build: all examples built
• Prettier check on changed files and git diff --check

The local environment could not install/start the matched browsers. The unchanged existing workflow has now completed on the fork for this exact head (5d9ca7d3b1ceb093f855bc2c4a4238eb3f6cd15d): CI run.

• All four jobs passed: Test, Browser compatibility, Build JS, and Build Docs.
• Chromium, Firefox, and WebKit: 114/114 cases passed, including the 18 new heading-level cases; retries remain disabled.
• Packed React 18 and React 19 declarations, bundle, styles, runtime, and public-ref output passed.
• The cross-version Markdown replay (0.35 input → packed 0.48 output → published 4.0.4/0.35 reader) passed.

This fork validation is separate from upstream CI approval. The PR remains draft.

[github-actions]

🎉 This PR is included in version 4.3.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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