#142: Make integer overflow semantics explicit

[zaguzovmaksim0-hue]

Closes #142.

The four integer atoms whose plain i16 arithmetic changed behavior between debug and release builds now use explicit two's-complement wrapping operations: wrapping_add, wrapping_sub, wrapping_mul, and wrapping_neg.

This preserves the release-mode arithmetic result while removing the debug-only overflow panic. It also fits the current Atom = fn(...) -> Option<Data> contract: None means that an input is not yet dataized, so using checked_* and returning None for arithmetic overflow would incorrectly make overflow look like a missing operand and could leave the emulator stuck.

Regression tests cover all four boundaries, including i16::MAX + 1 and negation of i16::MIN.

Validation on Android/Termux:

• cargo fmt -- --check — passed;
• cargo test --all-targets — passed;
• cargo clippy --all-targets --all-features -- -D warnings — passed;
• git diff --check — clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.11%. Comparing base (6025413) to head (2e9743d).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   94.09%   94.11%   +0.02%     
==========================================
  Files          14       14              
  Lines         711      714       +3     
==========================================
+ Hits          669      672       +3     
  Misses         42       42              

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