TLS: nonblocking BIO / off-loop OpenSSL (post-v1.2.0)

[robertbetts]

Context

lean-grpc v1.2.0 made h2c truly Async under Std.Async (#6, docs/async-io.md). In-process TLS (native/tls_ffi.c / SSL_read / SSL_write) remains blocking FFI wrapped via AsyncByteTransport.ofBlocking.

Goal

End-to-end Async (or off-loop) TLS so serveTls / connectH2 do not stall the UV loop during handshake or byte IO.

Options to evaluate

1. OpenSSL nonblocking BIO + integrate readiness with Std.Async / libuv
2. Off-loop worker pool for TLS read/write (keep BIO blocking off the UV thread)
3. Hybrid: async TCP + TLS terminate in sidecar only (already optional via LEAN_GRPC_TLS_PROXY)

Acceptance

• Documented Async TLS path (or explicit off-loop model) with no false “fully async TLS” claim until true
• Existing mTLS / peerIdentity AuthN continues to work
• CI coverage (tlsLoopback + at least one concurrent Async+TLS smoke if on-loop)

Follow-up to #6 / #9.

[robertbetts]

Implementation: #11 (v1.3.0) — off-loop worker model (issue option 2).

• H2.runOffLoop / AsyncByteTransport.ofBlockingOffLoop
• connectH2Async / serveH2Async / serveTlsAsync
• Sync connectH2 / serveTls remain pure blocking IO
• Docs: docs/async-io.md
• CI: asyncTlsLoopback

OpenSSL is still blocking FFI; this stops it from stalling the UV loop. Nonblocking BIO tracked as a follow-up after #11 merges.

Will close #10 when #11 lands on development.

[robertbetts]

Closed by #11 on development (v1.3.0 off-loop TLS). Nonblocking BIO: #12.