Emit absolute URLs when rewriting creative resources #915

Problem

Creative rewriting currently emits root-relative first-party proxy and click URLs (for example, /first-party/proxy?...). These URLs depend on the document origin and do not work reliably when the creative is rendered or consumed outside that origin context.

Proposal

When rewriting creative HTML/CSS, generate absolute first-party URLs using the configured publisher/base URL for proxy, click, and injected creative-script references where applicable.

Acceptance criteria

• Rewritten creative resource URLs are absolute URLs.
• Rewritten click-through URLs are absolute URLs.
• The configured first-party origin is used consistently.
• Tests cover HTML and CSS rewriting plus click URLs.
• Documentation examples reflect absolute rewritten URLs.

Notes

Preserve signed-query behavior and current URL-exclusion rules.