fix(cache): one cleanup thread per cache directory, not per manager (#453)

The display process ran three cleanup threads over one directory:

    14:22:59.954  display_controller        (the real manager)
    14:22:59.973  startup validation, run 1 (discarded)
    14:23:01.055  startup validation, run 2 (discarded)

Two of those managers existed only to read a directory path.
StartupValidator._validate_cache_directory built a whole CacheManager to
call get_cache_dir(), and validation runs twice -- once before the
plugin manager exists and again after. Each construction also probes
writability by writing and deleting .writetest on the card.

The discarded ones never went away. cleanup_loop closes over `self`, so
the thread keeps its manager alive: two objects that could never be
collected, waking every 24 hours to re-scan the same 9,000-file
directory. Nothing stopped them either -- stop_cleanup_thread had no
callers anywhere in the tree.

Two changes. The validator now takes the CacheManager the application
actually uses, which is also the more correct thing to validate; when
no caller supplies one it still builds its own, but stops the thread
afterwards. And CacheManager now tracks which directory it is sweeping,
so the second manager over a directory skips starting a thread at all.
That is the right granularity regardless of call sites: the sweep lists
a directory and deletes from it, so a second thread only duplicates the
scan. Ownership is released on stop, so a survivor can take over rather
than leaving the directory permanently unclaimed by a dead owner.

Measured directly, three managers over one directory: 3 threads before,
1 after.


Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/display_controller.py

@@ -90,7 +90,8 @@ class DisplayController:
         # Validate startup configuration
         try:
             from src.startup_validator import StartupValidator
-            validator = StartupValidator(self.config_manager)
+            validator = StartupValidator(self.config_manager,
+                                         cache_manager=self.cache_manager)
             is_valid, errors, warnings = validator.validate_all()
             
             if warnings:
@@ -258,7 +259,8 @@ class DisplayController:
             # Validate plugins after plugin manager is created
             try:
                 from src.startup_validator import StartupValidator
-                validator = StartupValidator(self.config_manager, self.plugin_manager)
+                validator = StartupValidator(self.config_manager, self.plugin_manager,
+                                             cache_manager=self.cache_manager)
                 is_valid, errors, warnings = validator.validate_all()
                 
                 if warnings: