Exponentiation and modulo in the calculator (closes #3)

The calculator now takes `^` (or `**`) for powers and `%` for modulo. Powers bind tighter than a sign on their left and group to the right, so `-2^2` is `-4` and `2^3^2` is `512`; `%` sits with `*` and `/` and takes the sign of the divisor. A small parser of our own replaces `go/parser`, which cannot express `^`; `go/constant` still computes.

A whole-number exponent is exact; a fractional one is computed in float64. Every number is held as a fraction under 4096 bits, and a float64 result must be a normal double, so one message cannot stall the bot; anything else gets "That needs a number too large or too small for me."

Disclosure: tiny values below about 2.2e-308, which `next` answered, are now refused.

Model: opus-5-5

internal/bot/bot.go

@@ -22,7 +22,7 @@ import (
 const DisplayName = "calc"
 
 // Welcome is sent to everyone whose contact request the bot accepts.
-const Welcome = "Send me arithmetic, such as 2 + 2 or 5 * 5/2, " +
+const Welcome = "Send me arithmetic, such as 2 + 2, 5 * 5/2, 2^10 or 7 % 3, " +
 	"and I will reply with the result."
 
 const (
@@ -221,10 +221,12 @@ func Reply(text string) string {
 			calc.MaxInputLength)
 	case errors.Is(err, calc.ErrDivisionByZero):
 		return "I cannot divide by zero."
-	case errors.Is(err, calc.ErrTooLarge):
-		return "The result is too large for me."
+	case errors.Is(err, calc.ErrOutOfRange):
+		return "That needs a number too large or too small for me."
+	case errors.Is(err, calc.ErrNoRealResult):
+		return "A negative number to a fractional power has no real result."
 	default:
-		return "I only understand arithmetic: numbers, + - * / and " +
-			"parentheses, such as 5 * 5/2."
+		return "I only understand arithmetic: numbers, + - * /, ^ for a power, " +
+			"% for a remainder, and parentheses, such as 5 * 5/2 or 2^10."
 	}
 }