fix: address F2/F4 review findings (leaks, qsort guard, err contract, compiler-default layering)

src/gen/args.c

@@ -388,8 +388,8 @@ static const char ARGS_PRE[] =
     "# Full argument parsing (todo 18): --help/-h, --version, --prefix/\n"
     "# --exec-prefix/--host/--build in both '=' and space forms, the\n"
     "# per-option --enable-<name>/--disable-<name> flags, and generic\n"
-    "# VAR=VALUE overrides (positional, e.g. `./configure CC=clang`, or\n"
-    "# exported by the invoking shell, e.g. `CC=clang ./configure`).\n"
+    "# VAR=VALUE overrides (positional, e.g. `./configure VAR=VALUE`, or\n"
+    "# exported by the invoking shell, e.g. `VAR=VALUE ./configure`).\n"
     "# Unknown --* options error. The loop only READS \"$@\", so the\n"
     "# config.log init right after this section still sees the complete\n"
     "# original invocation.\n"
@@ -414,7 +414,7 @@ static const char USAGE_HEAD[] =
     "    printf '%s\\n' '                        differs from --build -> cross-compiling'\n"
     "    printf '%s\\n' '  --build=TRIPLET, --build TRIPLET'\n"
     "    printf '%s\\n' '                        the platform the build is running on'\n"
-    "    printf '%s\\n' '  VAR=VALUE              override the variable VAR (e.g. CC=clang);'\n"
+    "    printf '%s\\n' '  VAR=VALUE              override a variable before substitution;'\n"
     "    printf '%s\\n' '                        both VAR=VALUE ./configure and'\n"
     "    printf '%s\\n' '                        ./configure VAR=VALUE work'\n";