M3: KVNode/KVStorage with arity dispatch, skip-scan keys, deleteTree, tree

Children are listed in subtree order (escaped segment + separator) so the
skip-scan stays O(children) even when sibling keys extend each other with
characters below the separator. Model-based property test vs a reference Map.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Qxbkp4tA7G5SCWjzWuxmE

src/index.ts

@@ -2,3 +2,4 @@ export * from './errors.js'
 export * from './core/path.js'
 export * from './core/encoding.js'
 export * from './kv/index.js'
+export * from './types/schema.js'