feat(node): expose the GRANDPA RPC so external verifiers can prove Orbinum finality

[nol4lej]

Why

While reviewing our testnet onboarding request (#1200), Hyperbridge asked us to wire the GRANDPA RPC into our node, they found it exposes no GRANDPA methods.

They were right: it was never wired in, so sc-consensus-grandpa-rpc wasn't even a dependency. Their prover needs grandpa_proveFinality to verify Orbinum finality.

Node-side only: no runtime change, spec_version stays at 11.

What

Wires sc_consensus_grandpa_rpc::Grandpa following the canonical pattern from polkadot-service/polkadot-rpc.

One thing needed care: SharedVoterState is now created once and shared with the voter. It used to be built inline at the voter, and a detached instance leaves grandpa_roundState reporting zeroed rounds instead of failing.

Pinned to sc-consensus-grandpa-rpc 0.44.0 — 0.45.0 would put two copies of sc-consensus-grandpa in the lockfile.

Verified on a dev node

grandpa_roundState returns live data (round: 87, real weights), which is what proves the voter state is shared.

Same binary, same flags, only the code differs:

baseline:    187 methods, 0 grandpa
with change: 191 methods  (+proveFinality +roundState +subscribeJustifications +unsubscribe)

Exactly four added, nothing removed. All work under --rpc-methods Safe, which the public endpoint uses, they serve finality proofs for already-final blocks, public by definition.

cargo test 12/12, clippy and fmt clean.