Every number a fraction under the 4096-bit limit (closes #3)

go/constant never rounds an integer, and x^0 returned one, so whole
numbers built from it escaped the 4096-bit limit:
(((2^0+2^0)^4096)^4096)^4096 asked for about 69 billion bits and
stalled the bot. A power now starts from the fraction 1, and a number
is exact only as a fraction whose numerator and denominator are below
4096 bits, which also refuses a literal that go/constant reads exactly
past the limit, such as 1e-1233. The bounded-work test holds the tower
and the other short inputs tried against the change.

Model: opus-5-5

README.md

@@ -172,18 +172,19 @@ container.
   computed as a double, so `2^0.5` is `1.4142135623730951`. A negative
   number to a fractional power is refused, as having no real result.
   Numbers are read as decimal, so `010` is ten. Input over 256 bytes is
-  refused and exact powers are capped, so a message cannot make the bot
-  do unbounded work. Whole numbers below 10<sup>21</sup> are written
-  exactly; other results in the shortest form that reads back as the
-  same double, in exponent notation from 10<sup>21</sup> up and below
-  10<sup>-6</sup>. Refused as too large or too small: any number whose
-  numerator or denominator reaches 4096 bits, wherever it appears, as
-  `go/constant` could hold it only rounded (`1e-1300 + 1`); a power
-  computed as a double whose base or result is outside the normal range
-  of a double, about 2.2e-308 to 1.8e308 in magnitude, where a double
-  keeps all its digits (`1e-400^0.5`); and a result other than zero
-  outside that range, as it is written through a double (`1e400`,
-  `2^-1400`).
+  refused, exact powers are capped, and every number is held as a
+  fraction, whole numbers too, under the 4096-bit limit below, so a
+  message cannot make the bot do unbounded work. Whole numbers below
+  10<sup>21</sup> are written exactly; other results in the shortest
+  form that reads back as the same double, in exponent notation from
+  10<sup>21</sup> up and below 10<sup>-6</sup>. Refused as too large or
+  too small: any number whose numerator or denominator reaches 4096
+  bits, wherever it appears, as `go/constant` rounds a fraction that
+  grows that large (`1e-1300 + 1`); a power computed as a double whose
+  base or result is outside the normal range of a double, about 2.2e-308
+  to 1.8e308 in magnitude, where a double keeps all its digits
+  (`1e-400^0.5`); and a result other than zero outside that range, as it
+  is written through a double (`1e400`, `2^-1400`).
 - **Failure is an exit.** If the chat client exits or the connection to
   it drops, the bot exits with an error and the container's restart
   policy starts both again. `SIGTERM` stops the bot, which stops the