refactor: single-source the native mode policy

verify.py and generate_pack.py must reach the same verdict on the same
file, and CLAUDE.md calls any divergence critical, but each spelled out
mode == 'existence' in its own words. Both now ask nativemode whether
the frontend reads the file's bytes, which is the one fact the rest
follows from, and a test holds the two answers together.

The BaseScraper contract the wiki asks contributors to implement had no
caller anywhere, so nothing proved compare_with_config, has_changes or
test_connection still worked.

scripts/nativemode.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+"""The one place that decides what a platform's native check actually does.
+
+verify.py reports coverage and generate_pack.py decides what goes in the ZIP.
+They have to reach the same verdict on the same file or the pack and the
+report describe different collections. The rule they share is small:
+
+    does the frontend read the file's bytes, or only look for the name?
+
+Everything downstream follows from that answer. RetroArch, Lakka and RetroPie
+call path_is_valid() and never open the file, so a declared hash contradicted
+by the local dump is a documentation error, the file still loads, and the
+builder ships it while the report flags the divergence. Batocera, Recalbox,
+RetroBat, RetroDECK, RomM, ROCKNIX, MiSTer and BizHawk compare a digest, so
+the same file would be rejected at runtime and shipping it would be shipping
+a known failure.
+
+Keeping the predicate here rather than in each caller is what makes
+"verify.py and generate_pack.py must agree" a property of the code instead of
+something a reviewer has to notice.
+"""
+
+from __future__ import annotations
+
+DEFAULT_MODE = "existence"
+
+#: Every verification mode a platform YAML may declare. Mirrors the enum in
+#: schemas/platform.schema.json; tests/test_audit_regressions.py pins them
+#: together so a new mode cannot be added to one without the other.
+MODES = ("existence", "md5", "sha1")
+
+#: The digest each content-checking mode compares. Modes absent from this map
+#: do not read the file at all.
+MODE_DIGEST = {"md5": "md5", "sha1": "sha1"}
+
+
+def normalize(mode: str | None) -> str:
+    """Return a declared mode, falling back to the schema default."""
+    return mode if mode in MODES else DEFAULT_MODE
+
+
+def reads_file_contents(mode: str | None) -> bool:
+    """Whether the frontend opens the file rather than just finding it."""
+    return normalize(mode) in MODE_DIGEST
+
+
+def digest_algorithm(mode: str | None) -> str | None:
+    """The hash this mode compares, or None when it compares nothing."""
+    return MODE_DIGEST.get(normalize(mode))
+
+
+def hash_mismatch_excludes_file(mode: str | None) -> bool:
+    """Whether a local dump contradicting a declared hash must be omitted.
+
+    True for the digest modes: the frontend would reject the file, so packing
+    it ships a known failure. False for existence: the frontend never looks,
+    and dropping the file over an upstream metadata error would remove
+    something that works.
+    """
+    return reads_file_contents(mode)