Deep Diff of Nested Data

Compare two nested structures (dicts, lists and plain values) and report what changed, like a JSON diff in a config review tool.

Implement deep_diff(a, b) returning a list of changes in traversal order:

• ("added", path, value), ("removed", path, value) and ("changed", path, old, new)
• Paths: dict keys are joined with . ("db.port"), and list indices use brackets ("servers[2]", "a.b[0].c"). The root path is "".
• Dicts: visit keys in sorted order (as strings) across both sides. Lists: compare index by index, then report extra items as added or removed.
• When the types differ (including dict vs list), or two plain values aren't equal, report "changed" at that path rather than recursing.

Also implement diff_summary(changes): a dict counting each kind, with all three keys present.