perf: read yaml through the c loader

Loading the emulator profiles is the most expensive step of every
command here, and all forty call sites used the pure-Python scanner
while libyaml sat unused in the same wheel. One shared yaml_load picks
the C loader when pyyaml ships it: the 375 profiles parse in 0.18s
instead of 1.39s, and verify --platform retroarch drops from 2.17s to
0.73s. The loader class is the same restricted one safe_load uses.

es_bios.xml was parsed straight from the network while install.py
already refused a document declaring entities; both now share one
guard. Scrapers reach it through a single path bootstrap in the
package rather than two ad-hoc ones.

scripts/check_buildbot_system.py

@@ -21,6 +21,7 @@ import urllib.error
 import urllib.parse
 import urllib.request
 from pathlib import Path
+from common import yaml_load
 
 log = logging.getLogger(__name__)
 
@@ -55,7 +56,7 @@ def load_tracked_entries(
         print("Error: PyYAML required", file=sys.stderr)
         sys.exit(1)
     with open(registry_path) as f:
-        data = yaml.safe_load(f) or {}
+        data = yaml_load(f) or {}
     entries: dict[str, tuple[str, str]] = {}
     for key, entry in data.get("data_directories", {}).items():
         url = entry.get("source_url", "")