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_test.go

@@ -16,6 +16,8 @@ func TestReply(t *testing.T) {
 	for in, want := range map[string]string{
 		"2 + 2":   "4",
 		"5 * 5/2": "12.5",
+		"2^10":    "1024",
+		"7 % 3":   "1",
 	} {
 		if got := bot.Reply(in); got != want {
 			t.Errorf("Reply(%q) = %q, want %q", in, got, want)
@@ -23,9 +25,11 @@ func TestReply(t *testing.T) {
 	}
 
 	for in, want := range map[string]string{
-		"hello": "I only understand arithmetic",
-		"1 / 0": "I cannot divide by zero.",
-		"1e400": "The result is too large for me.",
+		"hello":    "I only understand arithmetic",
+		"1 / 0":    "I cannot divide by zero.",
+		"1e400":    "That needs a number too large or too small for me.",
+		"1e-1300":  "That needs a number too large or too small for me.",
+		"(-8)^0.5": "A negative number to a fractional power has no real",
 		strings.Repeat("1+", calc.MaxInputLength) + "1": "That is too long for me",
 	} {
 		if got := bot.Reply(in); !strings.HasPrefix(got, want) {