fix(dpdk): clarify intentional RX notification result discard

[thweetkomputer]

GCC with fortified libc warns about the intentionally ignored read() result in the RX notification drain even when it is cast to void. Bind the result to a [[maybe_unused]] local so this best-effort operation compiles cleanly.

Keep the output-only eventfd counter uninitialized and document why: read() writes it, and packet readiness comes from the RX queues independently of its value. A failed drain retains the existing retry-on-wake behavior.

Validation on Linux ARM64:

• Compiled src/io/dpdk_backend.cpp with GCC 13 and Clang 18, using the configured DPDK/SPDK headers, _FORTIFY_SOURCE=3, -Werror, and -fsyntax-only; both passed without warnings.
• Compared GCC optimized assembly before and after; instructions are identical (only the input filename directive differs).
• pre-commit run --files src/io/dpdk_backend.cpp and git diff --check passed.

No hardware DPDK runtime test was run. Architecture documentation is unchanged because notification ownership, lifetimes, and failure behavior are unchanged.

[coderabbitai]

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