Write and read projects in the C locale, and keep the seed whole

gtkmm sets both the C and the C++ global locale from the environment, so
an unguarded stream writes "0,35" on a German desktop and groups 1920 as
"1.920" — neither of which the reader, or any other JSON reader, can take
back. Both the writer and the number parser now work in the classic
locale: the writer imbues its stream, and the parser uses from_chars
rather than stod, JSON having a point for a decimal mark whatever the
user's mark is.

The seed is read from the digits it was written as rather than from the
double they parsed into: a 64-bit seed has more bits than a double has
mantissa, and a seed that comes back off by a few is a project that no
longer renders its own picture.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/core_tests.cpp

@@ -2,11 +2,13 @@
 
 #include <array>
 #include <atomic>
+#include <clocale>
 #include <cstdio>
 #include <cstdlib>
 #include <filesystem>
 #include <fstream>
 #include <iostream>
+#include <locale>
 #include <mutex>
 #include <optional>
 #include <string>
@@ -671,6 +673,51 @@ int main() {
         }
         check(threw, "a missing project file throws");
 
+        // A project is JSON whatever the user's locale is. gtkmm sets both the
+        // C and the C++ global locale from the environment, so on a German
+        // desktop an unguarded stream writes "0,35" and a grouped "1.920" —
+        // which is not JSON, and the project could not be opened again.
+        const char* const commaLocales[] = {"de_DE.UTF-8", "de_DE.utf8", "fr_FR.UTF-8",
+                                            "fr_FR.utf8"};
+        bool decimalCommaTried = false;
+        for (const char* name : commaLocales) {
+            if (std::setlocale(LC_ALL, name) == nullptr) continue;
+            std::locale previous;
+            try {
+                previous = std::locale::global(std::locale(name));
+            } catch (const std::exception&) {
+                continue;
+            }
+            decimalCommaTried = true;
+
+            Project wide;
+            wide.width = 1920;
+            wide.height = 1080;
+            wide.seed = 17353667313003709502ULL;  // more bits than a double holds
+            wide.stages.push_back({effectByName("grain"), {{"grain_amount", 0.35}}});
+
+            const auto text = projectToJson(wide);
+            bool reread = false;
+            try {
+                const auto back = projectFromJson(text);
+                reread = back.width == 1920 && back.height == 1080 &&
+                         back.seed == wide.seed &&
+                         paramDouble(back.stages[0].values, "grain_amount", -1.0) == 0.35;
+            } catch (const EffectError&) {
+            }
+            check(text.find("0.35") != std::string::npos &&
+                      text.find("\"width\": 1920") != std::string::npos,
+                  std::string("a project is written as JSON under ") + name);
+            check(reread, std::string("a project written under ") + name + " opens again");
+
+            std::locale::global(previous);
+            std::setlocale(LC_ALL, "C");
+            break;
+        }
+        if (!decimalCommaTried) {
+            std::cout << "  skip a decimal-comma locale (none installed)\n";
+        }
+
         std::filesystem::remove_all(dir);
     }