feat: add RouteKind symbol brand to core types and define functions

[qantrepreneur]

What to build

Add a RouteKind symbol brand to the framework's route module system. definePage, defineHandler, and defineErrorPage stamp a RouteKind symbol on the config they return, enabling nominal type discrimination instead of structural checks ("template" in module).

• RouteKind = Symbol.for("sundayceo.routeKind") exported from core/index.ts
• PageModule type includes [RouteKind]: "page"
• HandlerModule type includes [RouteKind]: "handler"
• Error pages are branded "page" (they are pages from the pipeline's perspective)
• definePage, defineHandler, defineErrorPage stop being identity functions — they spread the input and stamp the brand
• RouteKind exported from index.ts

This is additive — existing named exports still work after this slice. The switch to export default and updated dispatch logic comes in the next slice.

Acceptance criteria

• RouteKind symbol defined and exported from core/index.ts
• PageModule type includes [RouteKind]: "page"
• HandlerModule type includes [RouteKind]: "handler"
• definePage stamps [RouteKind]: "page" on returned config
• defineHandler stamps [RouteKind]: "handler" on returned config
• defineErrorPage stamps [RouteKind]: "page" on returned config
• RouteKind re-exported from package index.ts
• Type tests updated
• All existing tests pass

Blocked by

None - can start immediately