[hackathon] feat: add signet-host-rpc crate with RpcHostNotifier

[prestwich]

Summary

• Add new signet-host-rpc crate providing RPC WebSocket-based HostNotifier implementation
• RpcHostNotifier<P>: follows host chain via newHeads subscription
• RpcChainSegment + RpcBlock: wrapper types for RPC-fetched blocks/receipts
• Ring buffer reorg detection via VecDeque block buffer
• Epoch-aware safe/finalized block tag caching
• RpcHostNotifierBuilder: fluent builder API
• PR [hackathon] refactor: decouple signet-node from reth via HostNotifier trait #100 review feedback: MissingBlock error variant, tighter field visibility, zero-guard on backfill_batch_size

PR 4 of 5 in the host context adapter refactor.
Precursor: #105 (independent of #106)

Test plan

• cargo clippy -p signet-host-rpc --all-features --all-targets
• cargo +nightly fmt

🤖 Generated with Claude Code

[prestwich]

[Claude Code]

Note: CI failures here are expected — test fixes land in #108 (fix/node-tests-host-notifier). PRs #104, #105, #106, #107, and #108 are intended to be merged as a unit; they are broken out into separate PRs for review isolation.