feat: complete fun/TODO special functions and add tests

[ulises-jeremias]

fun/TODO tracks outstanding special-function work:

• Untested: cheb, trig, erf, fib, gamma, digamma, hypot, mod_bessel, cgamma
• No tests for any of these

Task

1. Add tests for each untested special function
2. Verify numerical accuracy against known references
3. Remove fun/TODO once all functions have coverage

Files

• fun/cheb.v, fun/trig.v, fun/erf.v, fun/fib.v, fun/gamma.v, fun/digamma.v, fun/hypot.v, fun/mod_bessel.v, fun/cgamma.v
• fun/TODO