Keep starter links under Astro base - #112

Summary

This fixes the sub-path links and assets described in #105:

• prefix the framework favicon and Shiki stylesheet with Astro's configured base
• apply the same helper to starter sidebar links, breadcrumbs, pagination, header navigation, 404, and deprecated-version links
• add a regression contract covering the framework component and the canonical starter files

The root index route noted in the issue is handled separately in #111.

Verification

• node --import tsx --test test/subpath-template-contract.test.ts test/url.test.ts
• pnpm --filter nimbus-docs typecheck
• pnpm --filter nimbus-docs build
• pnpm --filter nimbus-starter-source typecheck
• pnpm --filter nimbus-starter-source build
• generated, scaffolded, installed, typechecked, and built a starter from the packed framework package
• built and browsed the starter at /spike; favicon, Shiki CSS, sidebar, breadcrumbs, header, and navigation all resolved under that path