fix(core): register tom_thumb, accept frame_hold in the test double, wire api_v3's managers

Three independent fixes found while validating every plugin on a 256x64 rig.

FontManager never registered tom_thumb even though assets/fonts/tom-thumb.bdf
ships with the core, so every plugin offering it logged "Font family
'tom_thumb' not found" (16 warnings per countdown render) and had to carry a
private loader to use a bundled font. Closes #524.

VisualTestDisplayManager.set_scrolling_state() lacked the frame_hold parameter
that DisplayManager gained, so any plugin passing it died with TypeError at
render time and failed every size. Nine plugins now make that call;
ledmatrix-stocks and ledmatrix-leaderboard were failing outright and the other
seven only passed because their scroll path was unreachable without data.
Closes #525.

api_v3 declared module-level config_manager/plugin_manager = None that nothing
ever assigned -- app.py sets the blueprint attributes, which the other 150+
call sites use. Three sites read the decoys, so /health reported the config
unreadable and the plugin system uninitialised (making "degraded" permanent and
unreachable-by-design) and /display/current fell back to a hardcoded 128x64 on
every rig. The decoys are removed rather than assigned, so a bare name is now a
NameError at test time instead of a silent None. The same function's first-call
uptime was computed from two separate clock reads and came out negative.
Closes #529.

Verified on the rig: both previously-failing plugins render, the tom_thumb
warnings are gone, /health reports "healthy" with all three checks passing, and
/display/current reports the real 256x64.

web_interface/blueprints/api_v3.py

@@ -78,8 +78,11 @@ def _scrub_git_remote_url(url: str) -> str:
     return url
 
 # Will be initialized when blueprint is registered
-config_manager = None
-plugin_manager = None
+# NOTE: the managers live on the blueprint object (app.py sets
+# api_v3.config_manager / api_v3.plugin_manager). Deliberately not
+# mirrored as module globals: a bare `config_manager` used to resolve to
+# a None that was never assigned, which silently disabled the /health
+# checks and made /display/current fall back to a hardcoded 128x64.
 plugin_store_manager = None
 saved_repositories_manager = None
 cache_manager = None
@@ -1598,11 +1601,16 @@ def get_health():
         }
 
         # Check web interface service
+        # Stamp the start time before measuring against it -- reading it with a
+        # fallback of time.time() and only assigning afterwards made the very
+        # first call subtract two separate clock reads, reporting a small
+        # negative uptime.
+        if not hasattr(get_health, '_start_time'):
+            get_health._start_time = time.time()
         health_status['services']['web_interface'] = {
             'status': 'running',
-            'uptime_seconds': time.time() - (getattr(get_health, '_start_time', time.time()))
+            'uptime_seconds': time.time() - get_health._start_time
         }
-        get_health._start_time = getattr(get_health, '_start_time', time.time())
 
         # Check display service
         display_service_status = _get_display_service_status()
@@ -1613,8 +1621,8 @@ def get_health():
 
         # Check config file accessibility
         try:
-            if config_manager:
-                test_config = config_manager.load_config()
+            if api_v3.config_manager:
+                test_config = api_v3.config_manager.load_config()
                 health_status['checks']['config_file'] = {
                     'status': 'accessible',
                     'readable': True
@@ -1633,9 +1641,9 @@ def get_health():
 
         # Check plugin system
         try:
-            if plugin_manager:
+            if api_v3.plugin_manager:
                 # Try to discover plugins (lightweight check)
-                plugin_count = len(plugin_manager.get_available_plugins()) if hasattr(plugin_manager, 'get_available_plugins') else 0
+                plugin_count = len(api_v3.plugin_manager.get_available_plugins()) if hasattr(api_v3.plugin_manager, 'get_available_plugins') else 0
                 health_status['checks']['plugin_system'] = {
                     'status': 'operational',
                     'plugin_count': plugin_count
@@ -2468,8 +2476,8 @@ def get_display_current():
 
         # Get display dimensions from config
         try:
-            if config_manager:
-                main_config = config_manager.load_config()
+            if api_v3.config_manager:
+                main_config = api_v3.config_manager.load_config()
                 hardware_config = main_config.get('display', {}).get('hardware', {})
                 cols = hardware_config.get('cols', 64)
                 chain_length = hardware_config.get('chain_length', 2)