refactor(app-router): extract app page dispatch

[NathanDrake2406]

What changed

This extracts App Router page request orchestration out of the generated RSC entry and into packages/vinext/src/server/app-page-dispatch.ts.

The generated entry now describes app shape and request wiring only: matched route, params, component tree builder, boundary render callbacks, middleware context, cache keys, and module loaders. The normal runtime module owns behavior: method policy, force-static/error setup, ISR cache read/regeneration, dynamic params validation, intercepting route responses, page element build error handling, lifecycle render delegation, and special-error fallback routing.

Why

This follows the principle that codegen should describe the app shape while normal modules implement behavior. The prior generated template mixed route-specific imports with a large page dispatch pipeline, which made orchestration harder to type, test, review, and evolve independently from route manifest generation.

Behavior tests

Added tests/app-page-dispatch.test.ts with response-level behavior coverage for the new dispatch module:

• production page cache HIT serves cached HTML instead of revalidating params or rendering
• unsupported page methods return the method policy response instead of rendering
• dynamicParams = false returns 404 for paths outside generated params
• intercepted RSC source-route payloads preserve middleware status and headers

These tests assert HTTP-visible outcomes: status, headers, and response body. They avoid asserting dispatch call counts or generated implementation details.

Next.js references

Next.js uses a similar separation between generated app-page shape and runtime page rendering modules:

• App page build template creates an AppPageRouteModule
• Generated app-page template delegates into a render closure rather than owning all renderer internals
• AppPageRouteModule is the runtime module boundary for app page rendering
• AppPageRouteModule.render() delegates to renderToHTMLOrFlight
• renderToHTMLOrFlightImpl owns the app render orchestration
• renderToHTMLOrFlight exposes the stable runtime entrypoint

Validation

• vp check --fix knip.ts packages/vinext/src/entries/app-rsc-entry.ts packages/vinext/src/server/app-page-dispatch.ts packages/vinext/src/server/app-page-request.ts tests/app-page-dispatch.test.ts tests/app-router.test.ts tests/entry-templates.test.ts tests/__snapshots__/entry-templates.test.ts.snap
• vp test run tests/app-page-dispatch.test.ts
• vp test run 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
• vp run knip --no-progress
• Commit hook reran staged-file checks and Knip successfully.

Risk coverage

Covered the main behavioral risks through dedicated dispatch behavior tests plus the App Router integration suite: dev/prod App Router rendering, RSC/HTML responses, ISR cache reads and writes, route handler separation, middleware header propagation, dynamic params validation, intercepting routes, not-found/forbidden/unauthorized boundaries, and generated-entry delegation assertions.

The generated-code tests intentionally assert only the delegation contract now. Runtime dispatch behavior is covered by tests/app-page-dispatch.test.ts, existing helper tests, and the App Router integration suite.

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commit: 1a713e6

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