Replace ffi-rzmq heartbeat device with poll loop

[kojix2]

• Replace ZMQ::Device with an explicit blocking poll/echo loop
• Treat ETERM and ENOTSOCK as normal heartbeat shutdown conditions
• Add a heartbeat shutdown hook so sessions can stop the loop before closing sockets
• Use zmq_ctx_shutdown for ffi-rzmq heartbeat cleanup when available
• Set zero linger on the ffi-rzmq heartbeat socket to avoid delaying shutdown

This addresses #357, where Ruby 3.4 test runs could finish successfully but hang at process exit when using the ffi-rzmq session adapter. The likely cause was the heartbeat thread staying blocked inside ZMQ::Device / zmq_proxy without an explicit cleanup path.

This pull request was prepared with assistance from Codex.

[Copilot]

Pull request overview

Replaces the ZMQ::Device-based heartbeat with an explicit poll/echo loop in the ffi-rzmq adapter and adds a dedicated shutdown_heartbeat hook so Session#close can unblock the heartbeat thread before tearing down sockets. This targets the Ruby 3.4 process-exit hang reported in #357 where zmq_proxy left the heartbeat thread blocked with no clean exit path.

Changes:

• Replace ZMQ::Device with a ZMQ::Poller-based echo loop in FfirzmqAdapter#heartbeat_loop, treating ETERM/ENOTSOCK/EINTR/EAGAIN as expected conditions.
• Add shutdown_heartbeat hook on BaseAdapter, ffi-rzmq (using zmq_ctx_shutdown when available) and test adapter; restructure Session#close to invoke it first and set @closed = true before stopping the heartbeat thread.
• Set ZMQ::LINGER=0 on the REP heartbeat socket and dedupe closed_sockets entries in the test adapter so the close-lifecycle test still asserts a single REP entry.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
lib/iruby/session.rb	Reorders close to mark closed, shutdown heartbeat, stop thread, then close sockets and adapter.
lib/iruby/session_adapter.rb	Adds default shutdown_heartbeat that just closes the socket.
lib/iruby/session_adapter/ffirzmq_adapter.rb	New poll/echo heartbeat loop, zmq_ctx_shutdown based shutdown hook, LINGER=0 on REP, renames type to type_symbol to avoid shadowing.
lib/iruby/session_adapter/test_adapter.rb	Dedupes closed_sockets and implements shutdown_heartbeat that flips @closed and closes the socket.

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