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

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