test: -- reaches neither the project args nor the script

`run -- record -- a` has to run the record script with the args `--`
and `a`: the first `--` ends the options, the second comes after the
script's name and is the script's. The project args keep only what
stood before `run`. And `run -- -x` has to look for a script called
-x, since after `--` an option-like word is an operand. Before the fix
the first `--` showed up in the project args, and `-x` joined it there
while the bare `run` listed the scripts and succeeded.

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

tests/Script/main.cpp

@@ -218,6 +218,29 @@ int main() {
         Check(fs::exists(binSentinel) && fs::exists(buildSentinel), "`clean` as a script argument deleted nothing");
     }
 
+    // `--` ends this tool's options and goes nowhere itself. In the
+    // project's args it gave the project a variant of its own, and every
+    // sibling that inherits the args one too. The word after it is an
+    // operand even when it looks like an option, here the script's name;
+    // after the name, a `--` is the script's like any other word.
+    {
+        fs::remove(scratch / "record.txt");
+        const std::int32_t code = RunWith({"crafter-build", project, "--flavour=x", "run", "--", "record", "--", "a"});
+        Check(code == 42, std::format("the script after `--` ran (got {})", code));
+        const std::vector<std::string> expected = {
+            "arg:--", "arg:a",
+            "projectArg:--flavour=x",
+            "debug:false",
+            "project:script-fixture",
+        };
+        const std::vector<std::string> record = ReadLines(scratch / "record.txt");
+        Check(record == expected, "`--` reached neither the project args nor the script");
+        if (record != expected) {
+            for (const std::string& line : record) std::println(std::cerr, "    recorded: {}", line);
+        }
+        Check(RunWith({"crafter-build", project, "run", "--", "-x"}) == 1, "after `--` a word that looks like an option names the script");
+    }
+
     // The first word after `run` is the name even when it is a verb.
     {
         fs::remove(scratch / "record.txt");