fix: materialize sanitized inline SVG styles

[chubes4]

Summary

• Materialize passive inline SVGs containing sanitizer-removed style elements as native core/image assets.
• Preserve local symbol/use artwork after sanitization, while retaining external sprite references as bounded fallbacks.
• Add parity and WordPress parse/serialize/render coverage for the nested-symbol path.

Root Cause

The native-image classifier evaluated the source DOM before sanitization. Puffin self-contained vectors commonly place a style element inside defs; although sanitization removes that style, the pre-sanitization classifier rejected the complete SVG and routed it to core/html instead of the materialized image path.

Verification

• php php-transformer/tests/parity/run.php
• composer --working-dir=php-transformer test
• php php-transformer/tests/integration/wordpress-site-plan.php (skipped locally because WP_TESTS_DIR is unavailable; the real WordPress assertion runs in configured test environments)
• git diff --check

Fixes #53

AI assistance

OpenAI GPT-5.6-sol via OpenCode general coding subagent. Used to inspect the corpus and transformer paths, implement the scoped classifier correction, and add regression coverage; Chris remains responsible for the change.