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");
 

tests/ShouldRoundTripAggregates/main.cpp

@@ -6,7 +6,9 @@
 // maps, a scoped enum, Required) through Value and its text, with no
 // hand-written serialization. Fields the input leaves out keep their default
 // member initializers, null clears an optional, and an enum also accepts its
-// integer. Guards the reflection that command arguments and results rely on.
+// integer. A float encodes as its shortest decimal, 0.1 rather than the
+// 0.10000000149011612 it widens to, which every float watch used to show.
+// Guards the reflection that command arguments and results rely on.
 import std;
 import Crafter.Debug.Core;
 using namespace Crafter::Debug;
@@ -74,6 +76,13 @@ int main() {
     Check(!partial.target.has_value(), "null clears an optional");
     Check(partial.mode == Game::Mode::Build, "an enum accepts its integer");
 
+    Check(Encode(0.1F).Dump() == "0.1" && Encode(-9.81F).Dump() == "-9.81" && Encode(4.0F).Dump() == "4.0",
+          std::format("a float reads as its shortest decimal ({}, {})", Encode(0.1F).Dump(), Encode(-9.81F).Dump()));
+    float tenth = 0;
+    DecodeContext tenthContext;
+    Decode(Value::Parse(Encode(0.1F).Dump()), tenth, tenthContext);
+    Check(tenthContext.Ok() && tenth == 0.1F && Encode(0.1).Dump() == "0.1", "and decodes to the same float; a double is unchanged");
+
     float nan = 0;
     DecodeContext nanContext;
     Decode(Encode(std::numeric_limits<float>::quiet_NaN()), nan, nanContext);