extractKeyMap doesn't follow $ref / recursive schemas — unbounded-depth trees silently lose keying #29

Problem

extractKeyMap (src/engine.ts) builds the pattern→key map by walking the schema's literal properties and items only. It does not resolve $ref or follow recursive schema definitions. So for a recursively-shaped tree — a node whose children array has the same keyed shape as the node — keying is only registered as deep as the schema is written out literally. Past that depth, arrays fall back to index-zip diffing and items()/diff() lose identity addressing silently (no error, just a wrong/cascading diff).

Example

A natural way to express an arbitrary-depth tree is a $ref:

{
  "type": "object",
  "properties": {
    "nodes": { "type": "array", "x-key": "id", "items": { "$ref": "#/$defs/node" } }
  },
  "$defs": {
    "node": {
      "type": "object",
      "properties": {
        "children": { "type": "array", "x-key": "id", "items": { "$ref": "#/$defs/node" } }
      }
    }
  }
}

extractKeyMap registers nothing for children (it never enters the $ref), so editing a grandchild produces an index-zip cascade instead of one identity-attributed op.

Today you must either:

• write x-key literally as deep as you ever nest ($['nodes'][*]['children'][*]['children'][*]…), which is impossible for unbounded depth; or
• pass { key } inline on each items() / diff() call at the level you're reading (this works, because inline key resolves against the concrete path at call time).

Possible directions

• Resolve $ref against $defs/# when walking the schema (handle self-reference without infinite recursion — detect cycles and register the key pattern as repeating).
• Or: support a recursive/wildcard pattern entry in the keyMap (e.g. a pattern that matches children at any depth) so a self-referential shape needs one registration.
• At minimum: document the limitation and the inline-key workaround in docs/engine.md.

Origin: surfaced while building a recursive tree editor on items() (branch claude/practical-shannon-jr19ie). Deliberately not addressed there — items()/diff() work fine with inline { key } per call.