docs(CubeNet): remove design document in CubeNet

[FakeLearne]

Design document should not appear in /CubeNet. This PR removed design document added in PR #1488.

[cubesandboxbot]

Review: docs(CubeNet): remove design document in CubeNet (#1579)

Verdict: Approve — no blocking issues. This PR deletes a single design document (CubeNet/docs/direct-neighbor-resolution.md, 101 lines) and touches no code, config, or build files.

What I verified in the base tree

• No dangling references to the deleted file. A repo-wide search for direct-neighbor-resolution finds only the PR diff itself; no README, docs index, CI config, script, or source comment links to the file. Deleting it leaves nothing broken.
• The design invariants survive in code. The deleted document's core rules (kernel neighbor table is the only MAC source, datapath reports / scanner schedules, gateway-MAC fallback never drops, CACHE_TTL/NEG_TTL lifetimes) are captured in the header comment of CubeNet/cubevs/direct_neigh_scanner.go and the scanner constants in the same file. The scanner implementation (StartDirectNeighScanner) and its call site (Cubelet/network/runtime/controller.go:316) are untouched by this PR.

Non-blocking observations

1. Information loss is partial, not total. The deleted doc is more detailed than the surviving code comments in one respect: the failure-model table (e.g. "Bounded blackhole ≤ scan interval + ARP RTT + NEG_TTL", "Policy-routed NIC → treated as unresolvable") and the parameter table (NEG_TTL = 1s, CACHE_TTL = 4s, trigger token bucket 100/s) are not restated anywhere. If the motivation is "design docs should not live in /CubeNet" rather than "this design is obsolete," consider relocating the document to a repo-level docs location instead of deleting it outright. This is a maintainer judgment call, not a defect — the code comments retain the operational essentials.

2. One comment loses its referent. CubeNet/cubevs/direct_neigh_scanner.go:22 reads "That stays within the documented 'cache staleness ≤ CACHE_TTL' failure model" — "documented" points at the file being deleted. The comment is still self-contained (it names the bound and constant), but if the doc is intentionally gone, that phrase could be reworded to reference the code invariant directly. Cosmetic; no action strictly required.

No inline comments were posted: the diff is a pure deletion with no lines to anchor to.

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AI-generated review. Reviewed against the base branch workspace only; PR head files were not assumed to be checked out.