v0.1.0

Added

• Initial scaffold: lexer, parser, AST, evaluator.
• Core arithmetic: + - * / mod ^ \ (root) ! (factorial) %, parentheses, unary minus.
• Standard math functions (radians): sin cos tan asin acos atan sinh cosh tanh asinh acosh atanh sec csc cot asec acsc acot sech csch coth asech acsch acoth sqrt ln log logn.
• Statistics functions: sum average product min max harmonic binom.
• CLI binary (excalc "2 + 2 * 3").
• CI: build/test on Linux, macOS, Windows; cargo fmt and cargo clippy checks.