docs(tech-design): expand auth.admin plan with compiler-verified type design

Detail the full type-safety design for the in-package admin handle: the
AdminOptions/AdminSurface/AdminGetterOf gate, GroupAdmin/LifecycleAdmin
interfaces, the single safe getter cast, and per-runtime + CDK + const-O
migration steps. Every conditional type was compiled under tsc --strict with
positive and @ts-expect-error negative cases (Appendices A/B); the proof lands
in-repo as admin.types-test.ts.

Author: harsh62