ringbuf: add UnsafeReader as zero-copy ringbuf reader

[jschwinger233]

ringbuf.Reader.ReadInto always copies sample bytes into user-owned memory. That is safe and convenient, but it becomes a visible bottleneck as sample size grows.

This PR introduces that path as a separate type (UnsafeReader) so the tradeoff is explicit: performance vs safety/concurrency guarantees.

APIs

Read

// Zero-copy Read — slice valid until Commit.
rec, err := unsafeReader.Read()
// Batch commit after processing N records.
unsafeReader.Commit()

ReadInto

// Zero-copy ReadInto — slice valid until Commit.
err := unsafeReader.ReadInto(&rec)
// Batch commit after processing N records.
unsafeReader.Commit()

ReadFunc

// Callback mode — sample is valid only during callback.
// Record is committed implicitly after callback returns.
rem, err := unsafeReader.ReadFunc(func(sample []byte, rem int) error {})

Benchmark Results

Benchmark is done by https://github.com/jschwinger233/bpf_ringbuf_zc_benchmark

  for i in {1..20}; do
    echo $((i * 128))
    taskset -c 2 ./bpf_ringbuf_zc_benchmark -event-size $((i * 128))
  done

Observed throughput (UnsafeReader.ReadInto vs Reader.ReadInto):

Event size	Speedup
128B	1.18x
1024B	3.61x
2048B	6.02x
2432B	6.39x

Trend: small records show modest gains; larger records show strong scaling, peaking at ~6.4x.

Related

#1968
#1930
#1915

[jschwinger233]

@ti-mo I drafted this PR to address the comments and move the discussion forward: #1968 (comment)