Powers and remainders in the calculator (closes #3)

internal/calc gets its own tokenizer and precedence-climbing parser in
place of go/parser, which cannot express a power operator; go/constant
still computes. ^ (also **) binds tighter than * / % and a sign on its
left, and groups to the right. % takes the sign of the divisor and is
exact on decimals. A whole exponent is computed exactly while the
result's numerator and denominator stay within 4096 bits; otherwise,
and for a fractional exponent, in float64. A remainder, or the sign of
a negative base's power, that depends on a value go/constant holds
rounded is refused as too large. The welcome text and the replies
mention the new operators; a negative base with a fractional exponent
gets its own reply.

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`