fix: propagate stack underflow error in continuation instead of masking with undefined

[TheUncharted]

Summary

• Bug Fixes
  • Enhanced error handling in the virtual machine to properly detect and report stack-related issues that were previously silenced, enabling faster identification of runtime problems and more reliable error reporting.

[coderabbitai]

📝 Walkthrough

Walkthrough

Error handling improvement in the process_continuation function within the VM. Replaces unwrap_or fallback with the question-mark operator to propagate stack underflow errors via Result instead of silently defaulting to Undefined. Enhanced comments clarify that empty stacks indicate VM bugs.

Changes

Cohort / File(s)	Summary
Stack Error Handling crates/zapcode-core/src/vm/mod.rs	Changed callback result extraction from unwrap_or fallback to fallible ? operator for explicit error propagation on stack underflow. Added clarifying comment about implicit PushUndefined+Return behavior.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	PR description is incomplete; it lacks detailed changes list and test plan validation despite having a summary section.	Complete the description by adding a detailed 'Changes' section listing key modifications and checking the test plan boxes to confirm unit tests and CI validation.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Title check	✅ Passed	The title accurately reflects the main change: replacing unwrap_or fallback with question-mark operator to propagate stack underflow errors instead of masking them with Undefined.

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✨ Finishing Touches

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[github-actions]

Benchmark Results

    Updating crates.io index
   Compiling zapcode-core v1.4.0 (/home/runner/work/zapcode/zapcode/crates/zapcode-core)
    Finished `bench` profile [optimized] target(s) in 16.14s
     Running unittests src/lib.rs (target/release/deps/zapcode_core-b075bfc4ac855948)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running benches/execution.rs (target/release/deps/execution-b4cebed678edeb20)
Timer precision: 15 ns
execution                  fastest       │ slowest       │ median        │ mean          │ samples │ iters
├─ array_creation          3.661 µs      │ 64.68 µs      │ 3.875 µs      │ 4.639 µs      │ 100     │ 100
├─ async_map_3             12.83 µs      │ 49.92 µs      │ 13.26 µs      │ 14.14 µs      │ 100     │ 100
├─ fibonacci_10            149.3 µs      │ 215.8 µs      │ 150.8 µs      │ 153.9 µs      │ 100     │ 100
├─ function_call           6.072 µs      │ 11.7 µs       │ 6.352 µs      │ 6.477 µs      │ 100     │ 100
├─ loop_100                87.87 µs      │ 100.5 µs      │ 88.61 µs      │ 89.22 µs      │ 100     │ 100
├─ object_creation         6.931 µs      │ 14.78 µs      │ 7.101 µs      │ 7.231 µs      │ 100     │ 100
├─ promise_all_3           9.496 µs      │ 24.1 µs       │ 9.823 µs      │ 10.08 µs      │ 100     │ 100
├─ promise_catch_resolved  6.159 µs      │ 17.18 µs      │ 6.353 µs      │ 6.511 µs      │ 100     │ 100
├─ promise_resolve_await   4.294 µs      │ 10.55 µs      │ 4.504 µs      │ 4.588 µs      │ 100     │ 100
├─ promise_then_chain_3    12.38 µs      │ 20.79 µs      │ 12.9 µs       │ 13.28 µs      │ 100     │ 100
├─ promise_then_single     7.524 µs      │ 13.56 µs      │ 7.788 µs      │ 7.932 µs      │ 100     │ 100
├─ simple_expression       3.23 µs       │ 5.531 µs      │ 3.355 µs      │ 3.415 µs      │ 100     │ 100
├─ string_concat           3.871 µs      │ 9.956 µs      │ 3.991 µs      │ 4.112 µs      │ 100     │ 100
├─ template_literal        4.229 µs      │ 9.412 µs      │ 4.366 µs      │ 4.438 µs      │ 100     │ 100
╰─ variable_arithmetic     4.322 µs      │ 10.21 µs      │ 4.5 µs        │ 4.596 µs      │ 100     │ 100