Configure TCP buffers and make flushing explicit

[wangxiao1254]

Summary

Make transport buffering configurable before connection setup and give flush() one precise meaning: drain local output.

Changes

• add tcp::SocketOptions with explicit send/receive buffer sizes;
• add bandwidth/RTT-derived sizing with a 256 KiB floor and power-of-two tiers;
• apply and verify options before listen/connect and before the TLS handshake;
• propagate the same policy to NetIO::make_sibling();
• expose socket options through TLSConfig and require defaults for adopted TLS sockets;
• remove the transport-level sync() hook;
• replace test synchronization with explicit marker round trips;
• report NetIO fflush failures;
• make TraceIO reject null transports, create trace files with mode 0600, flush traces before the wrapped channel, and report write/flush failures.

API migration

IOChannel::sync() is removed.

• Call flush() when the protocol needs to drain locally buffered output.
• Exchange an explicit protocol message when the protocol needs a peer barrier or round trip.

EMP-OT has a coordinated migration that replaces its remaining synchronization calls with explicit flushes.

Zero-valued socket options retain the operating-system defaults. Explicit buffer requests fail if the kernel cannot provide the requested minimum.

Tests

• clean Apple Clang 17 Release/Ninja build: 90/90 build steps;
• full CTest suite: 53/53 passed;
• focused test_netio, test_tlsio, and test_traceio: 3/3 passed.

Coverage includes buffer derivation, unavailable-buffer rejection, listener/client/sibling inheritance, TLS socket settings, adopted-socket rejection, explicit sibling round trips, send-only draining, ignored flush errors, TraceIO delegation, trace flushing, and file permissions.

Stack

This is PR 2 of 6 and targets fix/byte-bool-api. After PR 1 merges, this PR can be retargeted to main without changing its diff.