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>

interfaces/Crafter.Debug.Core-Codec.cppm

@@ -192,7 +192,20 @@ export namespace Crafter::Debug {
 
     template<std::floating_point T>
     struct Codec<T> {
-        static Value Encode(T value) { return Value(static_cast<double>(value)); }
+        static Value Encode(T value) {
+            // A float as the shortest decimal that reads back as it: 0.1f
+            // widened is 0.10000000149011612, which is what an agent would
+            // otherwise read for every float.
+            if constexpr (sizeof(T) < sizeof(double)) {
+                if (std::isfinite(value)) {
+                    std::array<char, 32> text{};
+                    const auto written = std::to_chars(text.data(), text.data() + text.size(), value);
+                    double shortest = 0;
+                    if (written.ec == std::errc() && std::from_chars(text.data(), written.ptr, shortest).ec == std::errc()) return Value(shortest);
+                }
+            }
+            return Value(static_cast<double>(value));
+        }
 
         static void Decode(const Value& value, T& out, DecodeContext& context) {
             if (value.IsNumber()) {