Refactor number.go to return errors instead of panicking

- Changed Number interface methods to return errors instead of panicking
- Float64() now returns (float64, error)
- Add, Sub, Mul, Pow, Compare, Bool() now return (Number, error) or (int, error)
- StringNum and Symbol now return errors for unsupported operations
- Added IsString() and IsSymbol() to Number interface
- Removed unused arithmetic.go file
- Updated operations.go, boolean_ops.go, hyper.go to handle errors
- Added constants registry (internal/rpn/constants.go) with built-in math constants
- Added constants_test.go with comprehensive unit tests
- Updated README.md with constants documentation

1 files changed, 5 insertions, 1 deletions

diff --git a/internal/rpn/variable.go b/internal/rpn/variable.go
index f3302df..6bde73d 100644
--- a/internal/rpn/variable.go
+++ b/internal/rpn/variable.go
@@ -26,7 +26,11 @@ func (o *VariableOperations) AssignVariable(stack *Stack, name string) error {
 	}
 
 	// Convert Number to float64 for variable storage
-	return o.vars.SetVariable(name, val.Float64())
+	valF, err := val.Float64()
+	if err != nil {
+		return fmt.Errorf("failed to get float64 value for variable: %w", err)
+	}
+	return o.vars.SetVariable(name, valF)
 }
 
 // UseVariable pushes a variable's value onto the stack.