docs(ai-explainer): document impl-diff pipeline step

[spalen0]

Docs-only follow-up to #226 and #227. The pipeline gained a structural implementation-diff step that wasn't reflected in `utils/llm/README.md`.

Changes

"Proxy Upgrade Detection" section (now "Proxy Upgrade Detection & Implementation Diff") covers:

• All three recognized upgrade selectors:
  • `upgradeTo(address)` and `upgradeToAndCall(address,bytes)` — called on the proxy itself
  • `upgradeAndCall(address,address,bytes)` — called on the OZ ProxyAdmin (proxy address comes from the calldata, not the tx target)
• The `ProxyUpgrade(proxy_address, new_implementation)` dataclass return type, with a note that `proxy_address` may differ from `target` (and the Telegram alert surfaces both).
• Selector short-circuit before calldata decoding so non-upgrade calls don't pay the Sourcify 4byte timeout.
• The structural diff via `utils/impl_diff.py`:
  • Functions added / removed / changed visibility or modifiers (overload-aware via name + arg-types).
  • Storage layout safety check, including OZ trailing-gap consumption recognition (`uintN[K] __gap` shrinks by exactly the number of inserted vars).
  • EIP-7201 namespaced storage detection — positional layout check skipped with a note.
  • Default-internal state vars (no visibility modifier) now captured; function locals excluded via brace-depth tracking.

Example assembled prompt updated to show the new `--- Proxy Upgrade ---` block content (Old/New impl labels, function add list, storage-layout verdict).

Module structure lists `utils/impl_diff.py`.

No code changes

Pure README update. CI will be fast.