* feat(app-router): promote segment reset semantics
Implements #726-CORE-11/12 from #726.
Add typed App Router segment state-key derivation, wire template/loading/error/access boundaries to semantic reset keys, and preserve pathname fallback semantics for legacy boundary callers.
Cover dynamic params, route groups, catch-all keys, template reset ownership, and boundary reset behavior.
* fix(app-router): treat empty boundary reset keys as absent
Empty segment identities can be produced for root, route-group-only, and default slot paths. Normalize empty reset keys to null so pathname fallback continues to clear caught boundary state.
* fix(app-router): scope route reset keys to branches
Use typed segment state identities and full visible route-state paths for route-wide wrappers so unrelated branches do not collide on the same leaf value.
Add hostile unit and browser tests for dynamic and static branch collisions.
Nathan Nguyen