feat: implement generic types on all column types

[Wyatt-SG]

Summary

• Add optional generic type parameter (<T extends BaseType = BaseType>) to all 25 simple type validators (t.string(), t.int32(), t.uuid(), etc.)
• Follows the existing pattern used by t.json<TShape>()
• Allows users to pass custom types (branded types, opaque types, literal unions) that flow through InferRow, InferEvent, and all inference utilities
• 100% backwards compatible- all generics default to their base type, zero runtime impact (compiled JS is byte-identical)

type UserId = string & { readonly __brand: "UserId" };

const events = defineDatasource("events", {
  schema: {
    user_id: t.string<UserId>(),  // InferRow → UserId
    name: t.string(),             // InferRow → string (unchanged)
  },
  engine: engine.mergeTree({ sortingKey: ["user_id"] }),
});

Verification

• pnpm run typecheck
• pnpm run build (emitted JS identical to before)
• pnpm -s vitest run src/schema/types.test.ts

[rmorehig]

Thanks for the PR! 🙏