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/diff_truth.py

@@ -16,7 +16,7 @@ import os
 import sys
 
 sys.path.insert(0, os.path.dirname(__file__))
-from common import list_registered_platforms, load_platform_config, require_yaml
+from common import list_registered_platforms, load_platform_config, require_yaml, yaml_load
 from truth import diff_platform_truth
 
 yaml = require_yaml()
@@ -27,7 +27,7 @@ def _load_truth(truth_dir: str, platform: str) -> dict | None:
     if not os.path.exists(path):
         return None
     with open(path) as f:
-        return yaml.safe_load(f) or {}
+        return yaml_load(f) or {}
 
 
 def _format_terminal(report: dict) -> str: