perf: define side-effect-free exports and add bundle-size regression tests

[gjbkz]

Summary

• remove observable import-time checker naming mutations and declare the package side-effect-free
• publish explicit core and individual-checker subpaths for npm and JSR
• add package-resolution, type-inference, tree-shaking, and bundle-budget regressions

Tests

• formatter, lint, and TypeScript checks
• focused package and bundle regression tests
• all Node unit tests and CI-equivalent c8 coverage suite
• complete CI-equivalent Bun suite
• build plus ESM and CommonJS package-entry smoke tests

Closes #440

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.69%. Comparing base (fa73337) to head (78824aa).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #447      +/-   ##
==========================================
+ Coverage   97.76%   98.69%   +0.92%     
==========================================
  Files          50       52       +2     
  Lines        2732     2834     +102     
  Branches      296      310      +14     
==========================================
+ Hits         2671     2797     +126     
+ Misses         61       36      -25     
- Partials        0        1       +1     

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