Rename single_node_default to single_node_exchange

[Copilot]

single_node_default is ambiguous—it sits next to single_node_reductions in definitions.py but doesn't convey that it's an exchange runtime. Renamed to single_node_exchange to match its type (SingleNodeExchange) and mirror the naming of its sibling.

• Renamed the module-level variable in decomposition/definitions.py
• Updated all 11 consuming files across model/common, model/atmosphere/diffusion, and model/atmosphere/advection (source + tests)

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[jenkins-apn]

Hi there, this is jenkins continuous integration...
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[jenkins-apn]

Hi there, this is jenkins continuous integration...
Do you want me to verify this patch?

[jcanton]

cscs-ci run default

[github-actions]

Mandatory Tests

Please make sure you run these tests via comment before you merge!

• cscs-ci run default
• cscs-ci run distributed

Optional Tests

To run benchmarks you can use:

• cscs-ci run benchmark-bencher

To run tests and benchmarks with the DaCe backend you can use:

• cscs-ci run dace

To run test levels ignored by the default test suite (mostly simple datatest for static fields computations) you can use:

• cscs-ci run extra

For more detailed information please look at CI in the EXCLAIM universe.

[jcanton]

cscs-ci run distributed

[msimberg]

This looks like a no brainer. Plenty of other related to changes that could be made, but I think this change is good to go on its own.