[feat][node] Add predecessor search round trip for join stage 1

[TheP2P (thep2p)]

Summary

• Add search_stage1_anchor, an async round trip that sends SearchByIdRequest directly to an introducer, bypassing this node's own (still empty, pre-join) local search, and correlates the reply via a fresh Waiter::AsyncSearch oneshot channel under a timeout.
• Extend the existing SearchByIdResponse handling to also resolve Waiter::AsyncSearch, alongside its existing blocking-caller Waiter::Search check.
• Direction is caller-supplied, not hardcoded, since which side to search from depends on where the introducer sits relative to this node (wired up in a later PR in this stack).
• New dedicated test (test_search_stage1_anchor_resolves), mirroring get_max_level's existing isolated-resolve test pattern.

Part of #90's join stage-1 linking, split into a PR stack: this is the second round-trip primitive (predecessor/successor search) after the address refactor and span-parentage cleanup.

Test plan

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test