docs(code): document deliberate duplicates instead of merging them

The audit surfaced several near-duplicate implementations that turned
out to be either deliberate forks or behaviorally different — merging
any of them would risk changing behavior on installed devices, so each
now carries an explicit comment stating the relationship:

- VisualDisplayManager: headless fork of DisplayManager; header now
  lists the ~15 mirrored methods and warns that DisplayManager changes
  must be mirrored.
- normalize_abbreviation: LogoDownloader's version (called directly by
  nine scoreboard plugins) replaces filesystem-unsafe characters;
  LogoHelper's strips spaces. Logo filenames on existing installs
  depend on both behaviors staying put.
- The two PluginTestBase classes: the shipped one is plugin-author
  API, the repo's own richer harness lives in test/plugins/ — now
  cross-referenced.

Also verified (no change needed): ConfigManager's backup/rollback
methods genuinely delegate to AtomicConfigManager, and SportsCore
already delegates _read_bdf_native_size to FontManager.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXb4mKcAkVaxkeTb3YnAdr

src/logo_downloader.py

@@ -118,7 +118,14 @@ class LogoDownloader:
     
     @staticmethod
     def normalize_abbreviation(abbreviation: str) -> str:
-        """Normalize team abbreviation for consistent filename usage."""
+        """Normalize team abbreviation for consistent filename usage.
+
+        Public API: sports scoreboard plugins call this directly.
+        NOTE: LogoHelper.normalize_abbreviation (src/common/logo_helper.py)
+        is a deliberately different variant (strips spaces, fewer character
+        replacements) — keep both behaviors stable; logo filenames on
+        existing installs depend on them.
+        """
         # Handle special characters that can cause filesystem issues
         normalized = abbreviation.upper()