test: add regression tests for turtle-singer note execution and counting behavior

[mahesh-09-12]

Summary

This PR continues strengthening regression protection for the turtle-singer.js engine by adding focused tests for the note execution pipeline and note counting logic.

A previous PR introduced regression tests covering lifecycle and pitch execution paths (killAllVoices, numberOfNotes, processPitch).
This PR extends that work by protecting additional execution behaviors related to note scheduling and counting.

This PR replaces #6113, which was closed to clean up commit history and remove unrelated changes introduced during rebasing.

No production code changes were made.

Covered Areas

1. noteCounter()

Adds regression tests protecting the internal counting mechanism used when evaluating note blocks.

Tests verify:

• Execution state restoration after counting
• suppressOutput is restored correctly
• justCounting stack integrity
• Turtle timing state remains unchanged

2. processNote()

Adds regression tests protecting the core note execution entry point.

Tests verify:

• BPM stack integrity during execution
• Correct fallback behavior when the BPM stack is empty
• Stage update is triggered after note processing

3. scalarDistance() Edge Cases

Adds coverage for important edge cases including:

• Equal note distance
• Reverse note ordering

Impact

• Improves functional coverage of turtle-singer.js
• Strengthens regression protection for note execution and counting behavior
• Ensures timing and counting behaviors remain stable during future refactors

PR Category

• Bug Fix
• Feature
• Performance
• Tests
• Documentation

[github-actions]

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

[mahesh-09-12]

@subhraneel2005 this PR continues the turtle-singer regression test work with a clean branch after closing the previous attempt that had unrelated commits during rebasing.