Report generic image asset references

[chubes4]

Summary

• Report every resolved local image asset reference in image_references, not only <img src>.
• Detect inline CSS style="...url(...)..." references and SVG <image href> / <image xlink:href> references.
• Preserve element, attribute, and context metadata through materialization-plan image asset rows.

Testing

• php -l src/AssetAnalysis/ReferenceAnalyzer.php && php tests/contract/run.php
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): gpt-5.5 via OpenCode
• Used for: Analyzing SSI matrix image-reference loss, implementing the generic Blocks Engine image asset reporting fix, and adding/updating contract and parity tests.