[feat][node] Add link request round trip for join stage 1

[TheP2P (thep2p)]

Summary

• Add send_link_request, an async round trip that sends GetLinkOp to a destination, asking it to adopt this node as its neighbor on a given side and level, correlating the reply via a fresh Waiter::Link oneshot channel under a timeout.
• Add the SetLinkOp match arm: the one and only write path for a lookup-table entry from a SetLinkOp, applying Core::try_link unconditionally whenever linked is present and level is in range (independent of whether a matching waiter exists), and separately resolving any pending Waiter::Link. An out-of-range level (peer-controlled) is rejected as RECOVERABLE, PEER-SAFE rather than propagated as a hard error.
• New dedicated test (test_send_link_request_resolves) plus the existing out-of-range-level regression test, both exercising the arm's write path against a real lookup table.

Part of #90's join stage-1 linking, split into a PR stack: this is the fourth and last round-trip primitive (link request), after the address refactor, span-parentage cleanup, predecessor search, and neighbor query. The orchestration that composes all four follows in a later PR.

Test plan

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