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/kv/index.ts

@@ -1,3 +1,6 @@
 export * from './driver.js'
 export * from './memory.js'
 export * from './sorted.js'
+export * from './node.js'
+export * from './indexes.js'
+export * from './children.js'