feat: deprecate unused plugin-facing methods for removal in 3.7.0 (#610)

35 methods on CacheManager, DisplayManager, FontManager and PluginManager
have no caller in core, the ledmatrix-plugins monorepo or the registry's
third-party plugins, but plugins live elsewhere, so they stay for one
release. src.deprecation.deprecated logs a warning (and emits a
DeprecationWarning) the first time each is called in a process, naming the
release that removes it. The list and replacements are in CHANGELOG and
PLUGIN_API_REFERENCE's new Deprecated APIs section; a test pins the set.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/plugin_system/plugin_manager.py

@@ -26,6 +26,7 @@ from src.plugin_system.schema_manager import (
     CORE_VEGAS_TUNING_KEYS, SchemaManager, normalize_legacy_booleans,
 )
 from src.common.path_safety import safe_path_component
+from src.deprecation import deprecated
 from src.common.permission_utils import (
     ensure_directory_permissions,
     get_plugin_dir_mode
@@ -698,6 +699,7 @@ class PluginManager:
         """
         return self.plugins.copy()
     
+    @deprecated("3.7.0", "check each plugin's enabled flag in plugins")
     def get_enabled_plugins(self) -> List[str]:
         """
         Get list of enabled plugin IDs.