sentinel: add multicast publisher worker

[snormore]

Summary of Changes

• Add the sentinel binary with a multicast publisher worker that polls for IBRL validators and automatically creates multicast publisher users onchain
• Validators are discovered via Solana RPC (getVoteAccounts + getClusterNodes) rather than the validator metadata API
• When --client-filter is set, the validator metadata service enriches with software client info; enrichment failure errors rather than returning unfiltered data
• Includes retryable RPC error handling with exponential backoff and Prometheus metrics
• Makes --is-publisher and --not-publisher mutually exclusive on the admin find command

This worker will be used as a library from the offchain repo sentinel instance. An e2e test in this repo will follow in a subsequent PR.

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	3	+702 / -0	+702
Scaffolding	3	+120 / -0	+120
Tests	—	+518 / -0	+518
Config/build	2	+51 / -0	+51

~35% test code; core logic is the poll cycle, Solana RPC reader, and RPC retry/error handling.

Key files (click to expand)

• crates/sentinel/src/multicast_publisher.rs — sentinel poll loop, ValidatorListReader trait, SolanaRpcValidatorListReader (Solana RPC impl), DZ ledger client for user fetch + publisher creation, metadata enrichment for client filtering
• crates/sentinel/src/error.rs — SentinelError type, retryable RPC error classification, rpc_with_retry with exponential backoff
• crates/sentinel/src/settings.rs — CLI args (--solana-rpc, --client-filter, --validator-metadata-url, --multicast-group-pubkeys)
• crates/sentinel/src/main.rs — binary entry point, metrics exporter, graceful shutdown
• crates/sentinel/src/lib.rs — module registration (error, multicast_publisher, settings)
• crates/sentinel/Cargo.toml — new dependencies (metrics, tracing, backon, tokio-util, url)

Testing Verification

• 37 unit tests passing covering: candidate selection, stake-based sorting, client filtering, enrichment error propagation, multi-group independence, creation error resilience, IBRLWithAllocatedIP user type
• cargo clippy and cargo +nightly fmt clean