Fix Windows crash: decode LuaJIT dump output as UTF-8, not the locale codepage

subprocess.run(..., capture_output=True, text=True) with no explicit
encoding falls back to locale.getpreferredencoding(False) -- the OS
default codepage. On Windows that's a legacy single-byte codepage
(e.g. cp1252), never UTF-8. When the LuaJIT dump contains a byte with
no mapping in that codepage, subprocess's internal _readerthread
crashes with an uncaught UnicodeDecodeError in a background thread; the
thread dies silently and the caller gets back stdout=None instead of a
string, crashing one line later with
AttributeError: 'NoneType' object has no attribute 'splitlines'.

Concretely, the Yellow-side imported dataset contains:
  "_ColosseumHeightText" -> "...6’8” tall!"
The right double quotation mark (U+201D) encodes in UTF-8 as E2 80 9D;
0x9D has no defined character in cp1252, so decoding as cp1252 fails
outright. Verified against the real imported dataset: the Red/Blue-only
dump has zero bytes outside cp1252's defined range; the Yellow dump has
exactly one, at this row.

UTF-8 is the actual encoding these dumps are produced in -- the driver
Lua sources are read/written as UTF-8 throughout this file, and LuaJIT
writes those source strings' bytes back out verbatim -- so passing
encoding="utf-8" explicitly at the three affected call sites
(run_loader, check_data_dump, dump_dataset) is a no-op on platforms
whose default codepage is already UTF-8 (Linux/macOS) and a correctness
fix on Windows.

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

tools/modkit.py

@@ -716,7 +716,7 @@ def run_loader(repo, mod_dir, findings, base="fixture", notes=None):
         driver_path = handle.name
     try:
         proc = subprocess.run([LUAJIT, driver_path], cwd=repo,
-                              capture_output=True, text=True, timeout=120)
+                              capture_output=True, text=True, encoding="utf-8", timeout=120)
     except FileNotFoundError:
         findings.append(Finding("MK100", "error",
                                 f"cannot run {LUAJIT} (install luajit or "
@@ -1056,7 +1056,7 @@ def check_data_dump(repo, path, base, rel):
     driver = DUMP_DRIVER % (lua_quote(path), lua_quote(vanilla))
     try:
         proc = subprocess.run([LUAJIT, "-e", driver], cwd=repo,
-                              capture_output=True, text=True, timeout=60)
+                              capture_output=True, text=True, encoding="utf-8", timeout=60)
     except FileNotFoundError:
         # the gate must fail closed: a missing interpreter is a broken
         # environment, not a clean mod
@@ -1416,7 +1416,7 @@ def dump_dataset(repo, base):
     handle.close()
     try:
         proc = subprocess.run([os.environ.get("LUA", "luajit"), handle.name],
-                              cwd=repo, capture_output=True, text=True)
+                              cwd=repo, capture_output=True, text=True, encoding="utf-8")
     finally:
         os.unlink(handle.name)
     if proc.returncode != 0: