feat: add zero graph diff command (#348-PR2)

Summary

Implement #348-PR2: Node-level "what changed" diff.

Adds a new zero graph diff --base <a> <b> --json command that computes a real changeset between two graph versions, keyed on the stable node identity established by #355.

JSON Output

{
  "schemaVersion": 1,
  "ok": true,
  "leftNodeCount": N,
  "rightNodeCount": M,
  "diffs": [
    {"kind": "added", "nodeId": "...", "nodeKind": "Function", "name": "foo"},
    {"kind": "removed", "nodeId": "...", "nodeKind": "Function", "name": "bar"},
    {"kind": "modified", "nodeId": "...", "field": "value", "leftValue": "300", "rightValue": "301"}
  ]
}

Acceptance Criteria

• A value edit shows as one modified entry (not add+remove)
• Add/delete classified correctly via stable node ID matching
• Read-only operation (cannot corrupt a graph)
• Conformance test added