Add built-in unit aliases for currency and generic units

[bpowers]

Summary

This change adds two new built-in unit definitions to the Simlin engine to improve unit handling for common use cases: currency (dollar) and generic units.

Key Changes

• Added "$" as a built-in unit with aliases "dollar", "dollars", and "$s" to standardize currency unit handling
• Added "unit" as a built-in unit with alias "units" for generic dimensionless unit references
• Added comprehensive test coverage for both new built-in units:
  • test_builtin_dollar_equivalences(): Verifies that $, Dollar, Dollars, and $s all resolve to the same unit and that they properly cancel out in ratios
  • test_builtin_unit_equivalences(): Verifies that Unit and Units both resolve to the unit unit

Implementation Details

The built-in units follow the same pattern as existing aliases like "person"/"people". They are registered in the builtin_units array in the Context::new_with_builtins() method, ensuring they are only added if the model doesn't already define units with those names. The tests confirm that case-insensitive matching works correctly and that unit cancellation operates as expected.

https://claude.ai/code/session_01SFHWDB5sKsuobFpi8wQhLP

[claude]

Code Review

Reviewed the changes in src/simlin-engine/src/units.rs.

No bugs found. The implementation correctly follows the established pattern for builtin unit aliases (matching person/people, year/years, etc.). Specific observations:

• $ as a unit name: The lexer already supports $ as a valid identifier character in LexerType::Units mode (via is_identifier_start / is_identifier_continue), so bare $ lexes and parses correctly.
• $s alias: Lexes as a single identifier $s in units mode — no ambiguity.
• "unit" builtin: Does not conflict with "dmnl" (the engine's dimensionless marker) or any other internal concept. The existing conflict-detection logic correctly skips builtins when the model already defines a unit with that name.
• Canonicalization: $ passes through canonicalize() unchanged (not affected by lowercasing, whitespace replacement, or period-to-middle-dot conversion).
• Test coverage: Both new tests verify alias resolution and unit cancellation, consistent with the existing test_year_years_builtin_alias pattern.

Verdict: Correct. The patch introduces no bugs and existing code/tests will not break.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.60%. Comparing base (3c07a42) to head (9e7cd6f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #396      +/-   ##
==========================================
+ Coverage   76.57%   76.60%   +0.02%     
==========================================
  Files         143      143              
  Lines       37026    37026              
==========================================
+ Hits        28353    28363      +10     
+ Misses       8673     8663      -10     

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