add missing test fixtures for string, math, and path methods

[cs01]

Summary

• Adds 5 new test fixtures covering previously untested methods:
  • string-case.ts — toLowerCase/toUpperCase
  • string-startswith-endswith.ts — startsWith/endsWith
  • string-charat.ts — charAt/charCodeAt
  • math-basic.ts — abs, max, min, floor, ceil, round, sqrt, pow
  • path-join.ts — path.join with absolute and relative paths
• Improves test coverage for core string operations and math functions

Test plan

• All 5 fixtures pass with both JS and native compiler
• npm run verify:quick passes

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