Answer exact results past the range of a double, such as 2^1200 (closes #16)

`2^1200` was refused although the calculator computed it exactly: writing a result went through a float64 and refused anything outside a double's range. An exact result under the 4096-bit limit is now written from its exact value, rounded to 17 significant digits past that range, and a fractional power of such a number is computed by first bringing its base into range with square roots taken from the exact value. Results computed in float64 must still be normal doubles; past the 4096-bit limit is still refused.

Disclosures: `1e-310` is answered again, reversing a call made in PR 11; a fractional power carries `math.Pow`'s rounding, which for a large base can reach the last digits shown.

Model: opus-5-5

internal/bot/bot_test.go

@@ -18,6 +18,7 @@ func TestReply(t *testing.T) {
 		"5 * 5/2": "12.5",
 		"2^10":    "1024",
 		"7 % 3":   "1",
+		"2^1200":  "1.7218479456385751e+361",
 	} {
 		if got := bot.Reply(in); got != want {
 			t.Errorf("Reply(%q) = %q, want %q", in, got, want)
@@ -27,7 +28,7 @@ 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":    "That needs a number too large or too small for me.",
+		"1e1300":   "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",