Storage: add automatic default EIP-7201 namespace policy

[Th0rgal]

Problem

#1730 originally covered automatic EIP-7201 namespaced storage. Verity now has opt-in storage_namespace, but it does not yet provide a default namespace policy for contracts that omit an explicit namespace.

Scope

• Choose the default namespace derivation rule for verity_contract storage.
• Preserve an explicit opt-out or compatibility mode for legacy/simple contracts if needed.
• Make generated slot roots stable and visible in artifacts/trust reports.
• Add diagnostics for ambiguous namespace configuration.
• Add smoke tests for default namespace generation and explicit namespace override behavior.