fix(php-transformer): chunk oversized projected stylesheets

[chubes4]

Closes #1549

Summary

• split generated CSS at rule boundaries below a conservative selector-record budget
• preserve selector order, nested conditions, and cascade order
• keep the original stylesheet path as an ordered @import loader for continuation chunks
• repeat required charset/namespace preambles without repeating source imports
• report exact emitted bytes and hashes for every generated chunk

Verification

• php tests/unit/stylesheet-selector-budget.php
• composer test (292 parity fixtures plus package-install proof)
• production-shaped import: 256,848-rule source projection loaded late matching control rules correctly after chunking
• browser-verified expected control geometry with zero console/page errors
• git diff --check origin/trunk...HEAD

AI assistance

OpenAI GPT-5.6 Sol via OpenCode measured the CSSOM/computed-style failure in Chrome, implemented generic selector-budget chunking and exact provenance, added deterministic regression coverage, and ran the focused, full, package, import, and browser verification under Chris Huber's direction.