fix(build): Introduce build pass to accomodate plugins with build-time code generation

[justinvdm]

Problem

Vite plugins that generate code at build time, such as Content Collections, typically use the buildStart hook to perform their work. The framework's directive scan, which is necessary to configure the main build, was running before this hook. This created a lifecycle mismatch: the directive scan attempts to resolve modules that are generated by these plugins. Because the scan ran before the plugins' buildStart hooks, the generated files had not been created yet. This causes the build to fail with a module resolution error, such as ERROR: Cannot read file ".content-collections/generated": is a directory.

Solution

This change introduces a "plugin setup pass" - a minimal, no-op Vite build that runs before the directive scan.

This pass is configured to be inert: it uses a temporary, empty file as an entry point and is set not to write any output. Its sole purpose is to trigger the buildStart hook for all configured plugins, ensuring that any necessary code generation is complete before the directive scan begins.

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