change(web): add .edgeKey for use + validation of complex .split cases 🚂

[jahorton]

When splitting SearchQuotientCluster instances, things can get a bit complex. "Later" instances are constructed recursively from "earlier" instances, and it's not in a tree-like fashion - there can easily be a directed graph leading form the search root to any given instance, with converging and diverging quotient-paths. If a split occurs during a "divergent" section, we need a strategy to faciltate de-duplication of split sections. .edgeKey is that tool.

It also provides a useful component of determining when two nodes may be considered "duplicates" of each other, something that can be very useful for unit-test assertions.

Build-bot: skip build
Test-bot: skip

[keymanapp-test-bot]

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• Android
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• Developer
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• iOS
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• Keyboards
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• Web
  • KeymanWeb Test Home - build : all tests passed (no artifacts on BuildLevel "build")

[mcdurdin]

LGTM, currently only used for unit tests?

[jahorton]

LGTM, currently only used for unit tests?

It sees use in the next PR, #15478, in a function only used in unit tests until #15031, at which point it is used within the standard codebase.