cli: add m2pack binary discovery helper

Resolve the m2pack-secure binary via M2PACK_BINARY env var or PATH,
raising ValidationError(m2pack_not_found) when neither works.

src/metin_release/m2pack_binary.py

@@ -0,0 +1,51 @@
+"""Resolve the m2pack binary for the m2pack wrapper subcommands.
+
+Resolution order:
+
+1. ``M2PACK_BINARY`` environment variable, if set, non-empty, and points at
+   an existing file.
+2. :func:`shutil.which` on ``m2pack``.
+
+If neither works we raise :class:`~metin_release.errors.ValidationError` so
+the CLI exits 1 with a clear, actionable message. Import of this module
+must never trigger filesystem access — all discovery is runtime.
+"""
+
+from __future__ import annotations
+
+import os
+import shutil
+from pathlib import Path
+
+from .errors import ValidationError
+
+
+ENV_VAR = "M2PACK_BINARY"
+
+_NOT_FOUND_HINT = (
+    "m2pack binary not found. Set the M2PACK_BINARY environment variable to "
+    "an absolute path, or make `m2pack` available on PATH. Build it from "
+    "https://gitea.jakubkadlec.dev/metin-server/m2pack-secure (see its "
+    "CMakeLists.txt)."
+)
+
+
+def resolve_m2pack_binary(env: dict[str, str] | None = None) -> Path:
+    """Return the resolved path to the m2pack binary or raise ValidationError."""
+    environ = env if env is not None else os.environ
+    override = (environ.get(ENV_VAR) or "").strip()
+    if override:
+        candidate = Path(override).expanduser()
+        if candidate.is_file():
+            return candidate.resolve()
+        raise ValidationError(
+            f"{ENV_VAR}={override!r} does not point at an existing file. "
+            f"{_NOT_FOUND_HINT}",
+            error_code="m2pack_not_found",
+        )
+
+    which = shutil.which("m2pack")
+    if which:
+        return Path(which).resolve()
+
+    raise ValidationError(_NOT_FOUND_HINT, error_code="m2pack_not_found")