fix: record each platform's own list version

install/<platform>.json wrote the constant "1.0" as the version of the
list it was built from. The manifest now carries the version the platform
YAML records, and generate_pack refuses a YAML without one.

inherits: no longer passes a parent's version to a child: Lakka was
published as RetroArch v1.22.2 in its manifest, pack name and site page.
lakka.yml records v6.1, the Lakka-LibreELEC tag its target list is read
at. The MiSTer scraper refuses a BiosDB without a timestamp instead of
writing an empty version.

A pack's manifest.json records list_version, so an install from the pack
alone knows it too. Manifests regenerated twice and compared: only
version and generated changed.

tests/test_save_root.py

@@ -243,7 +243,7 @@ class _Fixture(unittest.TestCase):
                              "required": False}]}
         for name in ("testra", "testnosave"):
             (self.platforms / f"{name}.yml").write_text(yaml.safe_dump({
-                "platform": name, "verification_mode": "existence",
+                "platform": name, "version": "2.1", "verification_mode": "existence",
                 "base_destination": "system", "cores": ["corex"],
                 "systems": {"test-console": system},
             }))
@@ -399,7 +399,7 @@ class AFolderPerSystem(_Fixture):
                              "required": False}]}
         for name in ("testbato", "testrb"):
             (self.platforms / f"{name}.yml").write_text(yaml.safe_dump({
-                "platform": name, "verification_mode": "existence",
+                "platform": name, "version": "2.1", "verification_mode": "existence",
                 "base_destination": "bios", "cores": ["corex", "corey"],
                 "systems": {"test-console": system},
                 # Two systems run CoreX, none runs CoreY.