Add deterministic asset pipeline

[mintyPT]

Summary

• Add a library asset resolver for local paths, file URLs, HTTP URLs, and data URLs
• Cache remote assets with deterministic SHA-256 keys and normalize resolved media to data URLs
• Surface asset diagnostics through template warnings and include resolved asset contents in template render cache keys
• Add focused media asset pipeline coverage

Verification

• npm run check
• Full test matrix left to GitHub CI per repository instructions

Fixes #35