chain: add an RPC method allowlist for chain requests

[mfw78]

Modules can call any JSON-RPC method via chain::request(chain_id, method, params); there is no allowlist ("No method allowlist, no re-encoding of params"). If the engine is configured with an RPC endpoint that exposes signing methods (eth_sign, eth_signTransaction, personal_unlockAccount), a malicious module could sign arbitrary transactions. Production endpoints are read-only in practice, so this is primarily operational. Fix: at minimum warn on dangerous methods, or document that operators must use read-only endpoints.

File: crates/nexum-engine/src/host/impls/chain.rs (approx lines 12-35)

[mfw78]

Heads-up from #153 (typed chain RPC seam, closes #103): the allowlist this issue adds now narrows WITHIN the structural ceiling of the ChainMethod enum rather than filtering raw strings. Two reconciliation points: docs/07's extra_allowed_methods was conceived as free-form widening (its example eth_createAccessList has no variant), so widening beyond the enum is now a code change and the config semantics should be defined as subset-selection; and the future identity-delegated signing path submits via the provider directly, not through this seam, so the enum does not block it - worth confirming when that lands.

[mfw78]

Resolved on develop by #153 (e4e9f58): chain::request now dispatches through a typed ChainMethod whose closed enum omits every signing and state-mutating method, so eth_sign, eth_signTransaction and personal_unlockAccount cannot be represented and never reach the provider (covered by the signing_and_mutating_methods_have_no_variant test). This is a structural allowlist that exceeds the issue's minimum ask. Closing manually: the repository default branch is main, so merge keywords against develop do not auto-close.