fix: preserve parentheses for same-precedence different operators

[cubic-dev-ai]

Summary

• Fix needBinaryOpArgParens to preserve parentheses when child and parent binary operators have the same precedence but are different operators (e.g., + vs -, * vs /)
• Previously, a + (b - c) was serialized as a + b - c, changing evaluation order from a + (b - c) to (a + b) - c
• Affects AppendString, Prettify, ExpandWithExprs, and Optimize code paths

Root Cause

needBinaryOpArgParens only compared operator precedence levels. When two operators shared the same precedence (e.g., +/- at priority 4, *///% at priority 5), parentheses were dropped if the sub-expression was a "simple leaf chain." This is only safe when the operators are identical (since they're left-associative), but not when they differ.

Fix

Added an isRight parameter to needBinaryOpArgParens to distinguish left vs right child position:

• Right child of left-associative op: require parens when child op differs (e.g., a + (b - c))
• Left child of right-associative op: require parens when child op differs (safety net for ^ precedence group)

Test plan

• Added 19 new test cases in TestParseSuccess covering all same-precedence operator combinations
• All existing tests pass
• go vet clean
• gofmt clean

🤖 Generated with Claude Code

---

Summary by cubic

Fixes binary expression serialization to keep parentheses when child and parent operators share precedence but differ, so re-parsing doesn’t change evaluation order.

• Bug Fixes
  • Updated needBinaryOpArgParens(arg, parentOp, isRight) to factor child side and require parens when differing operators at the same precedence would change associativity.
  • Keeps forms like a + (b - c), a * (b / c), a and (b unless c), and a ^ (b ^ c) intact across AppendString, Prettify, ExpandWithExprs, and Optimize.
  • Added 20 tests and updated example output to 100 * (disk_free_bytes{...} / disk_total_bytes{...}).

^{Written for commit 2d9fa2c. Summary will update on new commits.}

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.70%. Comparing base (8ba28d7) to head (2d9fa2c).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
parser.go	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
+ Coverage   89.67%   89.70%   +0.02%     
==========================================
  Files          11       11              
  Lines        2984     3010      +26     
==========================================
+ Hits         2676     2700      +24     
- Misses        210      211       +1     
- Partials       98       99       +1     

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[cubic-dev-ai]

No issues found across 3 files

[f41gh7]

LGTM