Refactor synthutils export structure to properly support Jest tests and coverage

[stutijain2006]

This PR fixes the export structure of synthutils.js so that Jest tests run correctly and coverage is properly recorded.

Earlier, although test cases were defined and running, the global test coverage was showing 0%, which indicated that functions were not being properly linked to the test environment. Some functions were also failing due to incorrect this binding and instance access.

Problem -

• Functions inside Synth were defined as instance methods.
• But they were being exported in a way that lost the correct this context.
• Tests were referencing class methods, while the actual logic depended on instance-level state.
• Some internal functions (like _createSampleSynth, setupRecorder, getTunerFrequency) were not properly accessible.
• Coverage report was almost 0% globally despite having defined test cases.

What was done-

1. Created a single Synth instance for export
   const synthInstance = new Synth();
   All exported functions are now bound to this instance:

loadSamples: synthInstance.loadSamples.bind(synthInstance),
getTunerFrequency: synthInstance.getTunerFrequency.bind(synthInstance),
...

2. Exported synthInstance for test control
   synthInstance.detectPitch = jest.fn(...)
   instead of modifying the classes directly.
3. Added required global mocks in test setup
   Some global helpers used inside synthutils were not defined in the test environment.
   Mocks were added to stabilize execution.

Final Result-

• All 86 test cases pass
• No runtime logic changed
• Proper instance binding
• Coverage is now correctly measured
• Global coverage is no longer showing near 0%
• synthutils coverage improved significantly

Importance of this PR-
Before-

• Tests existed
• But universal test was not importing this and showing 0%
• Some internal logic was not actually being validated correctly

After-

• Tests truly execute the intended logic
• Coverage metrics are accurate
• The module structure is cleaner and more maintainable

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[vanshika2720]

@stutijain2006 please run prettier on modified files.

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[kartikktripathi]

Try rebasing this, I believe you ran prettier on synthutils.js too but it still shows formatting problems, maybe rebasing will work.

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[stutijain2006]

@omsuneri This PR increases the test coverage for synthutils and I believe this is ready to merged. Please see to it.