Fix FORTRAN parser: CHARACTER length, assignments, DO WHILE, and more

- Fix CHARACTER type length parsing (len=N and (N) syntax)
- Fix assignment statement parsing (separate = from equality operator)
- Fix executable statement detection for assignments
- Fix SELECT CASE range syntax (case (2:5))
- Fix function RESULT clause parsing
- Fix type attributes parsing (intent(in) before ::)
- Fix DO WHILE statement detection and parsing

All 15 integration tests now passing (up from 6/15)

Author: MarsZDF