Add proxy-delegated chain extensions (IDs 16-27)

Add 12 proxy-delegated chain extension variants that allow a smart
contract with proxy rights to manage stake on behalf of a proxied
coldkey. Each handler verifies proxy authorization via zero-delay
proxy storage lookup before dispatching as the proxied account.

- verify_proxy: checks Proxies storage for matching delegate with
  correct ProxyType (Staking for most ops, Transfer for transfer_stake)
- 12 Proxy* FunctionId variants in types.rs with NotAuthorizedProxy output
- ink! contract bindings for all 12 proxy functions
- 17 proxy tests (5 auth verification + 12 per-operation success)
- 2 discriminant stability tests
- Updated wasm-contracts.md documentation

Author: LandynDev