cow: source the registry address from module [config]

[mfw78]

The module must learn the ComposableCoW registry address from its manifest [config], not a compiled constant. Ungated, can start today, and forms the first car of the convergence train.

Why

The converged ccow-monitor knows exactly one contract, the registry. Hard-coding cowprotocol::COMPOSABLE_COW couples the module to a single network and puts a cow-rs rev on the critical path. Old upstream Sepolia is deliberately dropped by the wire swap, and the constant itself is slated for deletion from cow-rs, so it must not survive as a fallback anywhere. Per-network manifest variants carry subscription addresses; the fork's deployments/networks.json is the operator's address authority, verified against the broadcast record.

Scope

• init parses registry from [config], mirroring AdapterConfig::parse.
• Remove cowprotocol::COMPOSABLE_COW as a default entirely: no Sepolia fallback, no fallback of any kind; the registry comes from manifest [config] only, and a missing or malformed registry key is a hard init error.
• MockHost tests for valid, absent, and malformed values.

Done when

• The module resolves the registry solely from manifest [config], with no default or fallback path.
• No code path uses COMPOSABLE_COW as the poll target or as a fallback; a missing registry key fails init loudly.
• MockHost tests cover valid, missing-key, and malformed-address cases.