Fix Split edge default and block mixed slot export

[kishore08-07]

Bug fix

Fixed bug

This PR fixes #183 by improving the behavior of Split operations in the Diagram Editor to make them more intuitive, consistent, and error-resistant.

Previously:

• Split edges defaulted to keyed, making edge labeling less discoverable for users.
• The editor allowed mixed output types (keyed and sequence) from the same Split node.
• Export was not proactively blocked for this invalid Split state.

Fix applied

• Default Split edge type preference now starts with sequence.
• Added Split export validation that rejects mixed keyed and sequence outputs from the same Split source.
• Export button is disabled when mixed Split slot types exist.
• Conflicting Split edges are highlighted in error color until the mismatch is resolved.
• Added regression tests for both behaviors.

Screencast.from.2026-03-18.14-11-01.webm

GenAI Use

We follow OSRA's policy on GenAI tools

• I used a GenAI tool in this PR.
• I did not use GenAI

Generated-by:
GPT-5.3-Codex

[mxgrey]

Thanks for opening this contribution. It's great to see the community identify pain points and put forward solutions.

I'm not certain, but I suspect we're going to see merge conflicts between this and #182, so just a heads up on that.

Added Split export validation that rejects mixed keyed and sequence outputs from the same Split source.

It's actually intentional that the Split operation supports a mix of keys and sequences. See the 💡Tip in this section of the handbook. We should revert this restriction.

Export button is disabled when mixed Split slot types exist.

I think it's dangerous for us to ever completely disable the export button. There may be times when a user wants to save a file in a bad state because they want to create a backup or need to exit the program, and they want to come back later to fix it.

I think a better approach would be a pop-up warning telling the user that the workflow is in a broken state and verifying that they want to save it, similar to the warning that a Save dialog usually gives if you try to save a file with a name that is already in use. For now we should revert this disabling behavior. The pop-up warning approach (or some other way to warn the user when the diagram is in a bad state) can be added as a separate PR.

[mxgrey]

Note this error that shows up while running the unit tests:

 error: crossflow_diagram_editor@0.0.7: Frontend has changed, run `BUILD_FRONTEND=1 cargo build` to update it

You'll need to run BUILD_FRONTEND=1 cargo build then commit and push the resulting changes.