fix: three errors that ended with the wrong exit code

Found compiling the environment-variable table for AGENTS.md:
- A REVIEW_GATES_MEM_RESERVE_MB that is no number threw
  std::invalid_argument, which nothing caught: the process aborted where
  it should end with a ValueError, exit 1. nan and inf were an unchecked
  cast. They raise ValueError / OverflowError now; a value past int64
  saturates.
- An env file that is not UTF-8: `run` ended with exit 1 where its
  preflight should stop it (exit 3), and `seats` parsed the raw bytes. Both
  read it as strict UTF-8 text now.
- A broker address with a port past 65535 was cut to 16 bits; it is an
  OverflowError now, uncaught (exit 1).

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

tests/Runs/main.cpp

@@ -358,6 +358,12 @@ echo "{\"type\":\"result\",\"subtype\":\"success\",\"is_error\":false,\"result\"
                               dump.is_object() ? Text(dump.at("stoppedAt")) : Read(base / std::format("{}.err", name))));
         }
 
+        // An env file that is not UTF-8: the preflight stops the run.
+        Write(base / "bad.env", std::string("\xff\n", 2));
+        const Process::Captured badEnv = run({"--no-reuse", "--no-salvage", "--env-file", (base / "bad.env").string()});
+        Check("preflight: an env file that is not UTF-8 stops the run, exit 3, with the decoder's own words",
+              badEnv.returnCode == 3 && badEnv.err.contains("preflight: 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte"), badEnv.err);
+
         // A stop between two gates: the next gate's pool starts aborted and
         // runs nothing of it.
         {