tracing: add the v2 shared-ring chunk protocol

Add the shared-memory ring that tracing v2 producers will write packet
fragments into, with a writer and a reader class on top of it:

- One 64-bit atomic holds both the write and the read position. Writers
  reserve positions with a lock-free CAS on it; the reader publishes
  read_pos once per drain pass.
- Every chunk starts with a 32-bit state word. The five states (Free,
  BeingWritten, Complete, RewriteRequested, RewriteAcknowledged) change
  hands only through exact-value CAS, so whoever loses a race sees the
  word that won.
- A Free word carries a 16-bit wrap count, so a delayed writer cannot
  claim a chunk for a position the reader already resolved. Only the
  reader ever writes Free.
- Writers append fragments to their chunk and publish by moving the
  state word. The reader may take the committed prefix of a chunk that
  is still being written; only the just-ended fragment can require
  relocation into a fresh chunk.
- Finishing a writer releases its cached chunk, so later fragments
  require a new reservation. This establishes the flush watermark
  boundary.
- Ring exhaustion follows BufferExhaustedPolicy: kDrop, kStall and
  kStallThenDrop. On Linux and Android a stalled writer waits on
  read_pos with a futex, under the same 30-second ceiling as v1.
- Tests cover the ABI encoding, reader/writer race outcomes, position
  and wrap-count rollover, and multi-writer stress against a concurrent
  reader.