fix: only pop instrumentNames if setTimbre pushed it

[Ady0333]

Summary

This PR fixes a stack imbalance in the Set Timbre block where nested blocks using the same instrument could remove the wrong entry from the instrumentNames stack, causing subsequent notes to play with an incorrect timbre.

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What changed

Added a small guard in setTimbre() to track whether the current block instance actually pushed a value to instrumentNames. The cleanup listener now pops the stack only when that push occurred.

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Why this change was needed

The push to instrumentNames was guarded by !includes(synth) to prevent duplicate entries. However, the listener always executed pop().

When identical Set Timbre blocks were nested, the inner block skipped the push but still executed the pop in its listener. This removed the outer block's entry, leaving the stack empty while still inside the outer scope.

As a result:

• last(instrumentNames) returned undefined
• Notes after the inner block played with the default synth instead of the intended timbre
• inSetTimbre was also cleared prematurely, disabling timbre effects for the remaining notes inside the outer block

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Scope

• Only affects setTimbre() in js/turtleactions/ToneActions.js
• No changes to note scheduling, synth loading, or block execution flow
• Existing behavior for non-nested timbre blocks remains unchanged

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Verification

• Tested nested Set Timbre blocks using the same instrument
• Confirmed outer timbre remains active after the inner block exits
• Notes play with the correct instrument throughout execution
• All tests pass with no regressions

---

Fix

if (!tur.singer.instrumentNames.includes(synth)) {
tur.singer.instrumentNames.push(synth);
pushed = true;
}

const __listener = () => {
tur.inSetTimbre = false;
if (pushed) {
tur.singer.instrumentNames.pop();
}
};

[Ady0333]

@walterbender and @vanshika2720 and @omsuneri , Please review this pr.

[github-actions]

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