change(common/models/templates): rework Trie predict method to utilize traversals 📚

[jahorton]

This PR follows from #11868 and #11869. While not originally part of #10973, it certainly leverages the same set of changes and will make a great base for future work toward user-dictionary support. (I did originally piece this together at the same time, though - just as a 'sibling' branch instead of as a descendant.)

Rather than keep two separate sets of logic for navigating through the Trie's nodes, this set of changes removes the old pattern, allowing prediction-lookup based on a prefix to operate via the properties defined on the LexiconTraversal structure. This essentially "dog-foods" the new pattern while keeping to DRY principles.

As mentioned in the description of #11868, this also aims to support merging / blending two (or more) different models together via their LexiconTraversal iterators in order to facilitate #11872. We'd want to blend the models together, then predict based on that blend.

To be clear, we already have these related items on our roadmap, in one sense or another:

• user dictionaries (feeding device-provided personal data into a predictions for a user)
• learning (so, learning the user's commonly-typed words and using them for corrections and predictions)

Both would likely be their own model that could be blended together in the manner described above.

User Testing

TEST_PREDICTIVE_TEXT: Using Keyman for Android or for iOS, verify that predictive text operates normally.

[keymanapp-test-bot[bot]]

User Test Results

Test specification and instructions

• ✅ TEST_PREDICTIVE_TEXT (PASSED): I tested this issue with the attached "Keyman-18.0.60-alpha-test-11870" build on the Android 14 Physical device and iOS(18.0.60-0.11870.11393). Here is my observation. (notes)

Test Artifacts

• Android
  • Keyman for Android apk
  • FirstVoices for Android 16.0 apk
  • FirstVoices Keyboards for Android apk
  • KeyboardHarness apk
  • Keyman for Android 16.0 apk
  • KMSample1 apk
  • KMSample2 apk
• Developer
  • Keyman Developer
  • Compiler Regression Tests
  • kmcomp.zip
• iOS
  • Keyman for iOS (simulator image)
  • FirstVoices Keyboards for iOS (simulator image)
  • TestFlight internal PR build version - 18.0.60 (0.11870.11422)
• Keyboards
  • Test Keyboards
• Web
  • KeymanWeb Test Home
• Windows
  • Keyman for Windows
  • FirstVoices Keyboards for Windows
  • Text Editor (32 bit)
  • Text Editor (64 bit)

[dinakaranr]

Test Results

• TEST_PREDICTIVE_TEXT (Passed): I tested this issue with the attached "Keyman-18.0.60-alpha-test-11870" build on the Android 14 Physical device and iOS(18.0.60-0.11870.11393). Here is my observation.

1. Installed the "Keyman-18.0.60.apk" file and gave all permissions to the application.
2. Checked the "Enable Keyman as system-wide keyboard" and set the keyboard as the default keyboard box on the settings page.
3. Open the Keyman app. Enable the "Predictions" and install the "Dictionary."
4. The banner suggested correct word lists while typing letters.
5. the word appears in double quotes on the banner if deleting letters.
6. the banner suggests a word while typing letters.
7. It works well in the Android and iOS environments. Thank you.

[keyman-server]

Changes in this pull request will be available for download in Keyman version 18.0.70-alpha