add optional TCP MD5 Signature RFC2385

[senid231]

TCP MD5 Signature (RFC2385)

Both clients accept tcp_md5_pass: to enable per-connection authentication via the TCP MD5 Signature option.
The kernel signs and verifies every TCP segment with MD5(key + segment + addresses/ports);
a peer with a mismatched or absent key has its segments silently dropped, so the handshake never completes.

[Copilot]

Pull request overview

Adds optional support for RFC2385 TCP MD5 Signature to the TCP transport, exposing it as a tcp_md5_pass: option for clients/transports and documenting/testing the behavior.

Changes:

• Add tcp_md5_pass: option plumbing in the transport base and implement TCP_MD5SIG setsockopt in JRPC::Transport::Tcp before connect.
• Add RSpec coverage for MD5-signed connections (match/mismatch, unsupported platform, overlong key) plus support helpers.
• Document the new option in README and CHANGELOG.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
spec/transport/tcp_spec.rb	Adds integration-style specs covering TCP MD5 signature success/failure paths.
spec/support/tcp_md5_helpers.rb	Introduces helpers to probe kernel support and configure an MD5-signing test server socket.
README.md	Documents tcp_md5_pass: and explains platform limitations/behavior.
lib/jrpc/transport/tcp.rb	Implements RFC2385 key installation on sockets via TCP_MD5SIG.
lib/jrpc/transport/base.rb	Adds @tcp_md5_pass option storage for transports.
CHANGELOG.md	Notes the new optional TCP MD5 signature feature.

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[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
jrpc	92%	81%	0	✔
Summary	92% (626 / 682)	81% (204 / 251)	0	✔