feat(relayer): introduce pallet-relayer with full relay infrastructure

[nol4lej]

Add meta-transaction relay support for shielded-pool operations, including the FRAME pallet, runtime-api, JSON-RPC server, client relay layer, node integration, and integration tests.

pallet-relayer (frame/relayer/)

• Storage: RelayerByAccount, PendingRelayerFees, MinRelayFee, AllowedSelectors
• Extrinsics: register_relayer, unregister_relayer, set_min_relay_fee, set_allowed_selectors, claim_fees
• RelayerInterface trait for cross-pallet access
• Weights, benchmarks, and inline unit tests (config, fees, registry)

pallet-relayer-runtime-api (frame/relayer/runtime-api/)

• RelayerRuntimeApi trait: is_relayer, pending_fees, registered_evm_address

pallet-relayer-rpc (frame/relayer/rpc/)

• JSON-RPC server exposing orbinum_relayerStatus

Client relay layer (client/rpc/)

• relay.rs: orbinum_relayShieldedCall — validates calldata selector whitelist, enforces fee >= max(min_fee, 2gasbase_fee), submits EVM transaction on behalf of the user
• signer.rs: EVM key loading and ECDSA signing utilities

Node integration (template/node/)

• --evm-relayer-key CLI flag for providing the relay signing key
• relayer_register.rs: background task that auto-registers the node as a relayer on startup when the key flag is present
• Relay RPC merged into the node RPC stack

Runtime wiring (template/runtime/)

• RelayerBlockAuthor adapter connecting pallet_authorship to pallet_relayer for fee attribution
• pallet_relayer::Config with DefaultMinRelayFee = 1e15 planck
• relay_config() Runtime API impl with day-0 selector fallback
• pallet_relayer added to construct_runtime at index 16

Tests

• ts-tests/test-relay-rpc.ts: integration suite covering disabled relay, validation errors, fee-slot regression, and tx lifecycle