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

docs/TODO.md

@@ -27,6 +27,9 @@ with no deprecation warning.
 
 # Completed Steps
 
+- 2026-09-28 Powers (`^`, also written `**`) and remainders (`%`) in
+  `internal/calc`, which now reads expressions with a parser of its own
+  in place of `go/parser`
 - 2026-09-28 Moved the command tree and the `run` and `version` commands
   from `cmd/simplexcalc/` into `internal/cli`; `cmd/simplexcalc/main.go`
   is now a single call to `cli.Main`