BREAK: merge collapse flags into one argument

[Flomber]

• Closes Clarify the interaction between strip_spin and collapse_graphs #350

⚠️ Interface changes

The boolean flags strip_spin and collapse_graphs on qrules.io.asdot() and qrules.io.asmermaid() are replaced by a single collapse argument that takes "spin", "topology", or None. The two booleans described mutually exclusive ways of combining transitions, but nothing in the signature said so: strip_spin=True, collapse_graphs=True was accepted by the type checker and only rejected at runtime. A single enumerated argument makes the exclusivity part of the interface, and the new CollapseMode alias (exported as qrules.io.CollapseMode) names the accepted values.

Old name	New name
strip_spin=True	collapse="spin"
collapse_graphs=True	collapse="topology"

🔨 Maintenance

• The branching that selects how transitions are combined lived twice, once in GraphvizPrinter and once in MermaidPrinter. It now lives in a single _labels.prepare_transitions() helper that both printers call, which also removes the attrs on_setattr validator that used to guard the mutually exclusive booleans.
• GraphvizPrinter is now kw_only, matching MermaidPrinter.

Squash commit messages

* MAINT: centralize transition collapsing in prepare_transitions
* MAINT: make GraphvizPrinter keyword-only

[Flomber]

I accidentally branched from mermaid-switch and thereby took all previous commits with me... 😞
Can we fix this, or do we have to create a new pull request for that?

[Flomber]

We once talked about implementing a validator for both MermaidPrinter and GraphvizPrinter.
We might want to talk about this again in person? 🤔