arrowbricks v5.1.1

• Fix a hole in 5.0.2's read_body: the up-front buffer reservation came from
  the untrusted Content-Length (capped at 1 GiB per download), and a failed
  reservation aborted the process, which the new allocator can turn into a
  crash under RLIMIT_AS or strict overcommit. It now reserves at most
  256 MiB, and falls back to Response::bytes() if even that fails.
• The allocator is behind a default-on large-block-alloc cargo feature and
  now requires Linux with glibc (it was only ever measured there; musl wheels
  are unaffected). A Rust crate that depends on arrowbricks_core with its
  own global allocator can opt out with default-features = false.
• Tests: a check that fails if the allocator is removed or mis-gated, read_body
  with and without Content-Length and with a short body, the Thrift inline
  LZ4 path with a zero-content frame, and import arrowbricks not loading
  asyncio (plus deferred submodules on 3.15).
• AGENTS.md and rust/.../alloc.rs: // SAFETY: comments with a lint to keep
  them, and design-invariant entries for the allocator, read_body and the
  lazy-import rule; the LZ4-loop entry now points at lz4_frame_decode_into.
• Removed the claim in 5.0.0 that the 3.15 suite passes with -W error: four
  tests fail intermittently under it from unclosed mock-server sockets.
• No API changes.