feat(framework): partition error pages in route scanner and codegen

[qantrepreneur]

This was generated by AI during triage.

Parent

• Error pages (404, 500) should not be reachable as regular routes #98

What to build

Update the route scanner and codegen to recognize error page files and separate them from regular routes.

Error page convention: A route file whose basename (minus extension) is a 3-digit HTTP status code in the 4xx–5xx range (e.g., 404.tsx, 500.tsx). Files like 200.tsx or 301.tsx remain regular routes. Error pages live in src/routes/ alongside regular routes — no new directory.

Route scanner (route-scanner.ts): scanRoutes should return error page entries separately from regular route entries. Error pages must not appear in the regular routes array.

Codegen (routes.gen.ts): The generated manifest should emit error pages as a separate export:

export const errorPages = {
  404: () => import("./routes/404"),
  500: () => import("./routes/500"),
};

Type codegen (framework.gen.d.ts): No new type registry needed — error pages already get type checking through defineErrorPage + TemplateRegistry.

Acceptance criteria

• Route scanner identifies error page files by 3-digit 4xx–5xx basename convention
• scanRoutes excludes error pages from the regular routes array
• Error pages returned as a separate data structure from the scanner
• Codegen emits errorPages export in routes.gen.ts with lazy imports keyed by status code
• Regular routes no longer include error page patterns (e.g., /404 is gone from the routes array)
• Files with non-error numeric names (e.g., 200.tsx, 42.tsx) remain regular routes
• Existing tests updated, new tests for the partitioning logic

Blocked by

None — can start immediately.