[Protocol] Add CheckNeighbor backpointer-probe RPC

[thep2p]

Summary

Periodic backpointer repair (Aspnes & Shah, arXiv:cs/0306043 — Algorithm 8, self-stabilizing backpointer repair) restores the invariant that if y is x's right neighbor at a level then x is y's left neighbor there (and symmetrically) — an invariant concurrent joins can transiently break. The repair sweep detects a broken backpointer by probing a peer, which the current RPC surface cannot express:

• CheckNeighbor — asks whether the receiver currently holds a given node as its neighbor at a (level, direction); returns a boolean.

This issue delivers only the probe transport: the request type, packet class, MiddleLayer dispatch, client sender, and a thin read handler. The periodic repair sweep that consumes it (Algorithm 8) — including the relink action taken when a probe fails — lands in a separate issue and is out of scope here.

In-scope handler behavior (exact): the CheckNeighbor handler calls the existing lookup-table isLeftNeighbor/isRightNeighbor (selected by direction) and returns a BooleanResponse. It is read-only; it performs no table writes and takes no repair action.

Scope

• src/main/java/underlay/packets/RequestType.java — add a CheckNeighbor enum entry.
• src/main/java/underlay/packets/requests/ — add CheckNeighborRequest (the candidate neighbor plus level and direction).
• src/main/java/underlay/packets/responses/ — reuse BooleanResponse. No new response class.
• src/main/java/middlelayer/MiddleLayer.java — add a receive() dispatch case and a client sender, following existing conventions (receiverId routing, lock gating).
• src/main/java/skipnode/SkipNodeInterface.java, src/main/java/skipnode/SkipNode.java — add the thin read handler described above, backed by isLeftNeighbor/isRightNeighbor. No table writes, no repair orchestration.

Acceptance Criteria

• RequestType has CheckNeighbor; a request class under requests/ carries the candidate neighbor plus level and direction, and it maps to BooleanResponse (no new response class).
• MiddleLayer.receive() dispatches it, and MiddleLayer exposes a client sender, matching existing sender/dispatch conventions.
• The handler returns true iff the receiver currently holds the candidate as its neighbor at the requested (level, direction), via the existing isLeftNeighbor/isRightNeighbor; it is read-only.
• No lookup-table writes and no repair/sweep orchestration are wired here.
• New public types and methods carry Javadoc (checkstyle clean).
• A test sends CheckNeighbor between two linked nodes over the mock underlay and asserts true for a held neighbor and false otherwise. make test and make lint pass. Total change including the test is well within 200 lines.

Dependencies

None. The periodic backpointer-repair sweep (Algorithm 8) that issues this probe and relinks on a failed check is a separate issue and out of scope here.

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Part of #33. Depends on: none — ready to start