Fix cross-proposal asset links

Resolve cross-proposal asset links before Zola sees prepared markdown.

Add proposal asset path resolution, rendered URL builders, and an
OnlyRenderPlan asset inventory so links can be validated before targeted
pruning removes omitted proposal content.

Rewrite static asset links to rendered relative URLs when targets are
available locally, and to public EIP/ERC asset URLs when targeted
rendering omits the target proposal. Keep selected asset markdown links
on the existing Zola @/... path, while omitted asset markdown links use
public page URLs.

Preserve query strings and fragments, leave fragment-only and raw HTML
links untouched, and skip already-generated Zola markdown so repeated
preprocessing remains idempotent.

Author: ritorhymes