Add TRIGSIN command for Pythagorean identity rewrites

[duncanita]

Summary

• Adds TRIGSIN command that rewrites cos(x)^2 to 1-sin(x)^2 using the expression rewrite engine
• Removes dead constant-folding rules that could never fire in the trig_sin rewrite context
• Fixes documentation to show actual output instead of overpromising sin²+cos² → 1 simplification
• Adds 8 tests covering recursion into subexpressions, multiple cos² terms, and negative cases

Test plan

• make sim compiles cleanly
• Full test suite passes: 3313 tests, 0 failures
• New tests verify: embedded cos², multiple cos² terms, cos³ passthrough, plain cos passthrough