Pack gap bit into size field's LSB

[devarajabc]

Reduce ringbuf_element_t from 8 bytes to 4 bytes by encoding the gap flag into the LSB of the size field. Since all payload sizes are padded to 8 bytes, the lower 3 bits of size are always zero, making it safe to reuse bit 0.

This shrinks the per-element header.

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Summary by cubic

Packed the gap flag into the size field’s LSB, shrinking ringbuf_element_t from 8 bytes to 4 bytes. This reduces per-element overhead and can improve cache locality.

• Refactors

  • Encoded gap via GAP_MASK (1) and masked size via SIZE_MASK; sizes remain 8-byte padded.
  • Updated read/write paths to set/test the gap bit and to mask size; fixed read_advance to use masked size.
  • Changed posix_memalign alignment to 8 bytes.

• Migration

  • Shared-memory layout changed; not compatible with buffers from previous versions.
  • Recreate ring buffers and roll out readers/writers together.