HTML API: Preserve PRE/LISTING newline skipping after seek

[sirreal]

Summary

• Preserve PRE/LISTING skipped-newline state when seeking back to a bookmarked text token.
• Restore skipped-newline state during seek() and keep it aligned when pending updates shift token offsets.
• Add bookmark and modifiable-text regressions for affected update and seek paths.

Testing

• HTML API and html5lib PHPUnit groups pass.
• PHPCS, syntax checks, and whitespace checks pass.
• codex review --base trunk.

Trac ticket: https://core.trac.wordpress.org/ticket/65372

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-5.5
Used for: Diagnosing the seek replay bug, test iteration, PR description cleanup, and code review.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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Props jonsurrell, sergeybiryukov.

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