feat(web): implement SearchCluster splitting and merging 🚂

[jahorton]

This PR serves to implement SearchCluster.split() and .merge() in full, meeting the following criteria:

• .split()
  1. If two or more paths result in an identical left-hand split result, that result should be deduplicated to a single instance.
     • This requires (live) use of the .isSameSpace() method from refactor(web): implement SourcePath merging 🚂 #15024, which is designed to identify such cases. It thus is no longer unit-test only.
  2. The results must meet the criteria for 'convergence' as described in change(web): track right-hand split index for input source of tokenized transforms 🚂 #15104 - even if that means returning multiple possible split tuples.
     • We must not group left-hand side paths (or right-hand side paths) that represent different subsets of the user's original keystrokes.
  3. The SearchCluster type should be leveraged for results where applicable - where two or more paths converge to the same total subset of keystroke input + codepoint length.
• .merge()
  • If two halves of a previous .split() operation are passed in, they should be fully remerged - into a single, refused SearchSpace segment based on the left-hand side's parent.

Build-bot: skip build:web
Test-bot: skip

[keymanapp-test-bot]

User Test Results

Test specification and instructions

User tests are not required

Test Artifacts

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