Refactor Astro runtime loading and Hono mode options

[aminevg]

Summary

• Refactors Astro manifest and page-module loading into dedicated core helpers.
• Updates dev rendering to source route assets from Astro route metadata and handle dev asset URL prefixing.
• Simplifies integration setup by removing the old renderer/script discovery helpers.
• Adds explicit Hono mode and root options, with stop() support on middleware.
• Expands tests to cover explicit production mode behavior and updated fixture wiring.

Testing

• pnpm build
• pnpm run typecheck
• pnpm run ci:biome
• pnpm test