fix(core): encode a float as its shortest decimal

A float was widened to double before encoding, so every float watch,
argument default and result read as the double nearest it: gravity
-9.81 showed as -9.8100004196167, a radius of 0.1 as 0.10000000149011612,
in `get`, `help` and call results alike. Agents read these, and copy
them back into `set` and `call`.

Codec<float> now encodes the shortest decimal that reads back as the
same float (std::to_chars), as a double. Decoding is unchanged, and so
is every value: 0.1 decodes to the float 0.1f came from. A double is
encoded as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ShouldResolveWatchPaths/main.cpp

@@ -51,7 +51,7 @@ int main() {
     });
     Watch("score", [] { return 1234; });
 
-    Check(registry().Resolve("physics.gravity").value.AsNumber() == static_cast<double>(-9.81F), "a variable watch reads the variable");
+    Check(registry().Resolve("physics.gravity").value.AsNumber() == -9.81, "a variable watch reads the variable, a float as its shortest decimal");
     Check(registry().Resolve("bodies[1].pos.y").value.AsNumber() == 4, "a path reads into a watched value");
     Check(registry().Resolve("bodies[1]").value.Find("id")->AsInteger() == 2, "an index reads an item");