fix(fact-checker): reconfigure stdio to UTF-8 on Windows

The `python -m mempalace.fact_checker --stdin` entry point reads non-ASCII
text through the system ANSI codepage (cp1252/cp1251/cp950) on Windows,
which mojibakes characters before claim-extraction sees them. Reconfigure
stdin/stdout/stderr to UTF-8 with `errors="strict"`, wrapped in try/except
so a replaced stream (Jupyter, test harness) logs a warning rather than
crashing the CLI.

Mirrors the same fix shipped for `mcp_server.py:main()` (#400) and
`hooks_cli.py:run_hook()` (#1280) -- this is the third and last
stdin-reading entry point in the package.

mempalace/fact_checker.py

@@ -303,11 +303,38 @@ def _edit_distance(s1: str, s2: str) -> int:
     return prev[-1]
 
 
+def _reconfigure_stdio_utf8_on_windows():
+    """Decode --stdin payload as UTF-8 on Windows.
+
+    Without this, Python defaults stdio to the system ANSI codepage
+    (cp1252/cp1251/cp950 depending on locale), which mojibakes
+    non-ASCII fact text before pattern parsing sees it.
+    """
+    import sys
+
+    if sys.platform != "win32":
+        return
+    for name in ("stdin", "stdout", "stderr"):
+        stream = getattr(sys, name, None)
+        reconfigure = getattr(stream, "reconfigure", None)
+        if reconfigure is None:
+            continue
+        try:
+            reconfigure(encoding="utf-8", errors="strict")
+        except Exception as exc:
+            print(
+                f"WARNING: Could not reconfigure {name} to UTF-8: {exc}",
+                file=sys.stderr,
+            )
+
+
 if __name__ == "__main__":
     import argparse
     import json
     import sys
 
+    _reconfigure_stdio_utf8_on_windows()
+
     parser = argparse.ArgumentParser(
         description="Check text against known facts in the MemPalace palace.",
         epilog="Exits 0 when no issues found, 1 when one or more issues detected.",

tests/test_fact_checker.py

@@ -286,3 +286,63 @@ class TestCLI:
         assert "similar_name" in out
         # Silence unused import warning.
         _ = (MagicMock, patch, fact_checker)
+
+    def test_reconfigures_stdio_to_utf8_on_windows(self):
+        """Windows fact_checker --stdin must decode payload as UTF-8.
+
+        Without this, Python defaults stdio to the system ANSI codepage
+        (cp1252/cp1251/cp950), which mojibakes non-ASCII text before
+        pattern parsing sees it.
+        """
+        import io
+        import sys
+
+        from mempalace.fact_checker import _reconfigure_stdio_utf8_on_windows
+
+        class _ReconfigurableStringIO(io.StringIO):
+            def __init__(self, initial_value=""):
+                super().__init__(initial_value)
+                self.reconfigure_calls = []
+
+            def reconfigure(self, **kwargs):
+                self.reconfigure_calls.append(kwargs)
+
+        stdin = _ReconfigurableStringIO()
+        stdout = _ReconfigurableStringIO()
+        stderr = _ReconfigurableStringIO()
+        with (
+            patch.object(sys, "platform", "win32"),
+            patch.object(sys, "stdin", stdin),
+            patch.object(sys, "stdout", stdout),
+            patch.object(sys, "stderr", stderr),
+        ):
+            _reconfigure_stdio_utf8_on_windows()
+
+        expected = {"encoding": "utf-8", "errors": "strict"}
+        assert stdin.reconfigure_calls == [expected]
+        assert stdout.reconfigure_calls == [expected]
+        assert stderr.reconfigure_calls == [expected]
+
+    def test_reconfigure_stdio_is_noop_off_windows(self):
+        """Linux/macOS already default to UTF-8 stdio -- helper must not touch streams."""
+        import io
+        import sys
+
+        from mempalace.fact_checker import _reconfigure_stdio_utf8_on_windows
+
+        class _ReconfigurableStringIO(io.StringIO):
+            def __init__(self):
+                super().__init__()
+                self.reconfigure_calls = []
+
+            def reconfigure(self, **kwargs):
+                self.reconfigure_calls.append(kwargs)
+
+        stdin = _ReconfigurableStringIO()
+        with (
+            patch.object(sys, "platform", "linux"),
+            patch.object(sys, "stdin", stdin),
+        ):
+            _reconfigure_stdio_utf8_on_windows()
+
+        assert stdin.reconfigure_calls == []