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

AGENTS.md

@@ -120,7 +120,7 @@ Do not weaken them.
 ```
 cmd/simplexcalc/    main(), a single call into internal/cli
 internal/bot/       startup, address setup, and the reply to a message
-internal/calc/      the arithmetic: go/parser and go/constant
+internal/calc/      the arithmetic: its own parser, and go/constant
 internal/cli/       cobra command tree: run and version
 internal/config/    viper-backed configuration; the abort-on-garbage rule
 internal/logger/    log/slog, JSON always