feat(host-rpc): add RpcAliasOracle with shared positive-result cache

[prestwich]

[Claude Code]

Summary

• Adds RpcAliasOracle<P> to signet-host-rpc — an RPC-based AliasOracle + AliasOracleFactory that queries EIP-7702 delegation bytecode via eth_getCode
• Shared Arc<RwLock<HashSet<Address>>> cache across all clones/oracles: once an address is confirmed as a non-delegation contract, subsequent lookups skip the RPC call
• Only positive results are cached (contract status is permanent); negatives are not cached since EOAs can become contracts later

Closes ENG-2079.

PR stack

#112 ← blobber abstraction ✅ merged
#113 ← config restructure ✅ merged

No remaining dependencies — this PR is ready to merge independently.

Test plan

• cargo clippy -p signet-host-rpc --all-features --all-targets — clean
• cargo +nightly fmt --check — clean
• cargo t -p signet-host-rpc — pass

🤖 Generated with Claude Code

[prestwich]

Code review

[Claude Code]

No issues found. Checked for bugs and CLAUDE.md compliance.

Several items were considered but scored below threshold:

• Delegation designator check uses starts_with rather than exact 23-byte length validation (low practical risk due to EIP-3541 EOF reservation)
• std::sync::RwLock guards are correctly scoped before .await but lack an explicit comment documenting the invariant
• No unit tests (consistent with the reth counterpart pattern)

None of these rose to the level of a blocking issue.

🤖 Generated with Claude Code

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