Preserve plaintext content on PHP 8.5

[chubes4]

Summary

Normalize explicitly closed plaintext elements before PHP 8.5 DOM parsing can absorb synthetic wrappers.

What changed

• Convert explicitly closed plaintext elements into escaped pre elements before DOMDocument parsing.
• Preserve encoded literal tags exactly once and exclude synthetic wrapper markup.
• Cover surrounding siblings, case-insensitive plaintext tags, and unchanged pre/code behavior.

How to test

1. Run composer --working-dir=php-transformer test; expect All contracts, unit tests, 235 parity fixtures, and package-install proof pass..
2. Run git diff --check; expect No whitespace errors are reported..

Compatibility

Explicitly closed plaintext elements now preserve only their bounded source content instead of PHP 8.5 parser leakage. Documents without plaintext and ordinary pre/code conversion are unchanged.

Evidence

• Reviewer-resolvable source evidence: Preserve plaintext content on PHP 8.5 DOM parsing #615
• diff_check: Passed
• php_transformer: Passed

AI assistance

• AI assistance: Yes
• Tool(s): Homeboy/OpenCode
• Model: openai/gpt-5.6-terra
• Used for: Reproduced the PHP 8.5 DOM behavior, drafted the normalization and contract coverage, and verified the final candidate with GPT-5.6 Sol; Chris reviewed and owns the change.

Source relationships

• Closes Preserve plaintext content on PHP 8.5 DOM parsing #615