feat: add --declared-only and the events schema

--declared-only keeps the entries the platform's own list declares
(cores: null), each slot settled to the platform's copy with no core
alternative left behind. Install, --check, --dry-run, --events,
--list-systems, --system, --region, --emulator, --target,
--installed-cores, --manifest and --from-pack apply it. --core,
--list-cores, --list-emulators, --list-platforms, --list-targets,
--standalone-copies, --restore and --tui refuse it, and the interface
does not open by itself with it. On Batocera it gives 353 files of
14531, the list batocera-systems checks.

Event decision: no schema bump. A `declared` key on `file` would add a
key to an existing kind, which schema 1 forbids. A new kind, `declared`,
follows `check` with the same counts over the platform's own files and
their dests in file-event order. Readers already skip a kind they do
not know (rbevents.py and the interface reader were checked), so
EVENT_SCHEMA stays 1.

An unknown --system is answered with the systems the list files its
key file under: --platform retroarch --system snk-neogeo now suggests
arcade, which holds neogeo.zip, instead of snk-neogeo-cd. neogeo, mvs
and aes follow the same route. Never taken, only suggested. A system
title the manifest carries (SNK Neo Geo CD) is taken for its id when
no other system shares it and no platform name reads the same.

schemas/install-events.schema.json is written from EVENT_KEYS by
scripts/generate_event_schema.py. validate_schemas.py reports a stale
file, and tests compare it with EVENT_KEYS and validate a real run.
Bootstrap pins recomputed.

scripts/validate_schemas.py

@@ -554,6 +554,23 @@ def _semantic_install_checks(path: Path, manifest: dict) -> list[str]:
     return out
 
 
+def _event_schema_errors() -> list[str]:
+    """install-events.schema.json is EVENT_KEYS written out, and a valid schema."""
+    import generate_event_schema  # noqa: PLC0415
+
+    path = generate_event_schema.OUTPUT
+    label = path.relative_to(ROOT).as_posix()
+    if not path.is_file():
+        return [f"{label}: missing, run python scripts/generate_event_schema.py"]
+    if path.read_text(encoding="utf-8") != generate_event_schema.render(
+        generate_event_schema.build_schema()
+    ):
+        return [f"{label}: differs from EVENT_KEYS in install.py, run "
+                "python scripts/generate_event_schema.py"]
+    _validator(path.name)
+    return []
+
+
 def unchecked_formats() -> list[str]:
     """Formats the schemas declare that this jsonschema install cannot check.
 
@@ -598,6 +615,7 @@ def main() -> int:
     errors.extend(_repository_layout_in_profiles())
     errors.extend(_save_directory_blocks())
     errors.extend(_registry_contract())
+    errors.extend(_event_schema_errors())
 
     if not args.source_only:
         database_path = ROOT / "database.json"