refactor: introduce createAppRscHandler

[NathanDrake2406]

Summary

This is stacked on #986

This introduces createAppRscHandler() and moves the top-level App Router RSC request lifecycle out of the generated entry and into a normal typed runtime module.

The generated entry now stays focused on app shape:

• route/module imports
• generated manifests and route maps
• thin closures that describe how to dispatch matched pages, route handlers, and server actions

The runtime helper now owns request behavior:

• dev origin checks and unified request context setup
• basePath / trailing-slash normalization
• prerender endpoints
• config redirects and rewrites
• middleware sequencing
• metadata and public file dispatch
• server action handoff
• pages fallback / not-found fallback
• post-response next.config.js header application

I also trimmed dead surface from the new seam by removing top-level handler options that were not actually consumed there (allowedOrigins, maxActionBodySize). Those remain owned by the action helpers, which is a better fit for the “codegen describes shape, normal modules implement behavior” split.

Why

app-rsc-entry.ts had accumulated too much request orchestration. That made the generated output harder to reason about, harder to test directly, and too easy to grow in the wrong direction.

This refactor keeps codegen responsible for describing the app-specific graph while moving stable behavior into importable runtime code with focused tests.

That layering is closer to how Next.js is structured:

• the template builds the route module shape: packages/next/src/build/templates/app-page.ts#L126-L142
• the route module exposes the runtime boundary and delegates to rendering logic: packages/next/src/server/route-modules/app-page/module.ts#L85-L169
• request/render orchestration lives in normal runtime code, not the template: packages/next/src/server/app-render/app-render.tsx#L2568-L3063
• the exported render entrypoint is a thin boundary over that runtime: packages/next/src/server/app-render/app-render.tsx#L3077-L3095

Tests

• vp check packages/vinext/src/entries/app-rsc-entry.ts packages/vinext/src/server/app-rsc-handler.ts tests/app-rsc-handler.test.ts tests/app-router.test.ts tests/entry-templates.test.ts
• vp test run tests/entry-templates.test.ts -u
• vp test run tests/app-rsc-handler.test.ts tests/app-page-dispatch.test.ts tests/entry-templates.test.ts tests/app-router.test.ts tests/app-page-render.test.ts tests/app-page-request.test.ts tests/app-page-cache.test.ts

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commit: db09233