R-492: delete the always-empty cfg.Paths.HDDPath (field, env binding, every fallback)

Readers (report builder, health check, /api/system, web primaryHDDPath, metrics collector,
AutoDiscoverStoragePaths' fallback parameter) now use the storage registry only. An old
controller.yaml still carrying paths.hdd_path keeps loading (non-strict YAML), pinned by
TestR492_OldConfigWithHDDPathStillLoads.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/monitor/healthcheck.go

@@ -76,7 +76,7 @@ func RunHealthCheck(cfg *config.Config, cpuCollector *system.CPUCollector, stora
 
 	debug := cfg.Logging.Level == "debug" && logger != nil
 
-	hddPath := cfg.Paths.HDDPath
+	hddPath := "" // R-492: the global paths.hdd_path is gone; the registry is the only source
 	if len(storagePaths) > 0 {
 		hddPath = storagePaths[0].Path
 	}