Answer fractional powers of exact numbers past a double's range (closes #16)

A fractional power turned its base into a double first and refused a
base outside the normal range of a double, so (2^1200)^0.5, (2^1024)^0.5,
(2^-1200)^0.5 and 1e400^0.5 were refused although each answer is an
ordinary double. Such a base is now brought into that range by square
roots taken from its exact value in a big.Float, at most three under the
4096-bit limit, with the exponent doubled for each, and only a result
that is not a normal double is refused. The tests give these powers and
the edges of the range their values, and the bounded-work test covers a
base that needs three roots. The README's refusal sentence and example
are updated.

Model: opus-5-5

docs/TODO.md

@@ -28,7 +28,8 @@ with no deprecation warning.
 # Completed Steps
 
 - 2026-09-29 Exact results past the range of a double, such as `2^1200`,
-  are written to 17 significant digits instead of being refused
+  are written to 17 significant digits instead of being refused, and a
+  fractional power of such a number, such as `(2^1200)^0.5`, is answered
 - 2026-09-29 `POST`, `GET` and `DELETE` on a chat's webhooks, under
   `/api/v1/chats/{id}/webhooks`, kept in `$DATA_DIR/webhooks.json`
 - 2026-09-29 `GET` and `POST /api/v1/chats/{id}/messages`: a chat's