Publish and subscribe from any Julia task

[tsaubergine]

Finishes the Julia pub/sub story: Goby.publish() and Goby.subscribe() now work from any task, on every layer, rather than only from Main.

Two commits, publish then subscribe.

Publish

check_and_publish threw an AssertionError naming the task for any layer other than INTERTHREAD unless it was called on Main. It now forwards those publications to the task that owns the C++ application over the interthread channel that task already has — which is what a C++ thread's InterProcessForwarder does with its inner interthread transporter.

publish_forward_interprocess is renamed publish_forward_cxx: the layer is passed through untouched, so INTERMODULE works the same way.

Subscribe

Subscriptions on the layers the C++ side owns were registered from whichever task called subscribe(), and every message came back to Main regardless of who had asked for it (the TODO in receive_forward_interprocess).

• check_and_cxx_subscribe() hands the registration to the application-owning task as a :cxx_subscribe channel message, so Goby.cxx_subscribe is always called on the task that owns the application.
• cxx_subscriber_tasks records which task asked for each (layer, scheme, type, group), and receive_forward_cxx delivers there. Anything unregistered still goes to Main, as before.

This also fixes a latent bug in interprocess_callbacks: the leaf was a single Function per key and was written without a lock, so two tasks subscribing to the same group kept only the last-registered callback. It is now keyed by task as well, behind the same lock as the delivery map, and receive_dereferenced runs only the callbacks belonging to the current task.

Known cost

An interprocess publication from a task waits up to one cxx_channel_check_frequency tick (default 10 Hz) before it goes out. That is documented in the README so an application with latency-sensitive traffic knows to raise it.

Tests

src/test/julia/test_app.jl gains testsets covering both registries directly — two tasks subscribing to one group, a task replacing its own callback, and the default-to-Main routing. Moving an actual message needs a portal, so that part is covered by the example rather than here. 29 assertions, passing.

Verified end-to-end against a live gobyd with the companion goby3-examples change (GobySoft/goby3-examples#48): reports flow publisher-task -> interthread -> subscriber-task -> interprocess, reaching both a separate subscriber process and the publishing application's own task subscription, exactly once each.

Docs

New "Multi-Threaded Julia Usage" section in share/goby/Goby.jl/README.md, and a note in doc250_languages.md that Julia — unlike Python — supports multi-threaded applications.

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