ringbuf: add zero-copy read with deferred consumer advancement

[orishuss]

Summary

Adds ReadZeroCopy and Commit to ringbuf.Reader, enabling zero-copy reads from the mmap'd ring buffer with batched consumer position advancement.

Motivation

ReadInto copies every record from mmap into a user buffer and advances the consumer position per-record via atomic store. In high-throughput scenarios (100K+ events/sec), the memmove and per-record atomics become a measurable CPU cost.

This change allows callers to:

1. Read records directly from the mmap'd region (no copy)
2. Process multiple records
3. Advance the consumer position once via Commit

Changes

• Reader.ReadZeroCopy(rec *Record) error — sets rec.RawSample to a slice of the mmap'd ring, does not advance consumer position
• Reader.Commit() — single atomic store to release all consumed space
• readRecord refactored to delegate to readRecordZeroCopy + copy + advance (no duplication)
• ReadInto and ReadZeroCopy share poll logic via readWait helper
• Windows: real zero-copy via ringReader delegation (same as Linux)
• Tests: single read, multi read with discards, commit releases space, deadline, no-op commit, benchmark

API

// Zero-copy read — slice valid until Commit.
err := reader.ReadZeroCopy(&rec)

// Batch commit after processing N records.
reader.Commit()

Existing Read/ReadInto semantics are unchanged.

Related

• ringbuf: add zero-copy consumer APIs #1915 (iterator-based zero-copy)
• ringbuf: add zero-copy reader callback API #1930 (callback-based zero-copy, per-record advancement)

This PR differs by deferring consumer advancement, eliminating per-record atomic stores in addition to the copy.