Add Observables.jl / Makie compatibility extension

[bvdmitri]

Summary

Adds RocketObservablesExt, a package extension that loads automatically when both Rocket and Observables are available (Julia ≥ 1.9) and provides a bidirectional compatibility layer between Rocket.jl and Observables.jl — the reactive primitive behind the Makie ecosystem.

Per the package-extension rules, the layer only adds methods to functions that already exist in either package (extensions cannot export new names). It works through two existing entry points:

• Rocket → Makie — overloads the Observables.Observable constructor:
  • Observable(behaviorsubject) / Observable(recentsubject) seed from the subject's current/recent value.
  • Observable(initial, source) is the generic form for any subscribable, subject, or operator pipeline.
  • A small NextActor forwards each Rocket emission into the Observable (observable[] = value), firing Makie's listeners. The Rocket subscription's lifetime is tied to the Observable via a finalizer.
• Makie → Rocket — as_subscribable + on_subscribe! make any Observables.AbstractObservable a valid Rocket source, so subscribe! and the full operator catalogue (map, filter, …) work directly on Makie observables. The current value is emitted immediately on subscription (mirroring BehaviorSubject semantics); teardown detaches the listener via Observables.off.

Is it a true drop-in?

Yes, with one explicit Observable(source) conversion — not a literal pass-through. Makie gates reactivity on isa(x, Observables.AbstractObservable), which a Rocket type can't satisfy (the abstract type lives in a weak dependency). Converting to a genuine Observable is the robust path and is exactly what the extension mechanism allows.

Changes

• ext/RocketObservablesExt.jl — the compatibility layer (methods only).
• Project.toml — [weakdeps], [extensions], Observables compat, and test extras/target.
• docs/src/integrations/observables.md (registered in make.jl) — a comparison section plus executed-in-CI Rocket→Makie and Makie→Rocket examples. Adds CairoMakie + Observables to the docs environment.
• test/ext/observables.jl (+ runtests.jl) — covers both directions and teardown, guarded for Julia ≥ 1.9.

Verification

• Full test suite passes (12256 tests, including the new extension tests).
• Strict docs build (warnonly = false) succeeds; both Makie figures render as PNG.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.40%. Comparing base (0b09967) to head (b9c4505).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #68      +/-   ##
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+ Coverage   78.20%   78.40%   +0.19%     
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  Files         119      120       +1     
  Lines        3799     3829      +30     
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+ Hits         2971     3002      +31     
+ Misses        828      827       -1     

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