v0.8.0 — group-commit write batching

Added

• ConnOptions.GroupCommit — opt-in write batching (group-commit). When a
  HEADERS writer finds another writer queued on the write lock, it defers its
  flush so the next holder batches both frames into a single tls.Conn.Write —
  fewer TLS record encrypts + socket writes under high per-connection stream
  concurrency. It is synchronous (the deferring writer waits for the convoy
  flush and returns its error, so per-frame write-error semantics are preserved
  — no async contract change), adaptive (a strict no-op with no contention;
  a lone request is never delayed), and off by default. Measured on one TLS
  h2 conn at 64 concurrent streams: 1.73 frames/flush, ≈−42% write syscalls,
  ≈+26% req/s; the win survives poseidon's dense-multiplexing pool (≈+30–35% at
  1–8 conns). Motivated by a review of ozontech/framer; see
  docs/WRITE_BATCHING_INVESTIGATION.md.
  Note: framer's net.Buffers/writev mechanism gives zero benefit through
  crypto/tls.Conn; plaintext frame coalescing is the only TLS-viable lever.