Platform adapter: rename System_Essentials, add Plugin/unraid/adapter.sh, wire call sites

- Rename unRAID_Essentials/ → System_Essentials/ (git detects as rename)
- Add Plugin/unraid/adapter.sh: 13 platform_*() functions providing OS-agnostic API
  for storage health, service management, mover, user scripts, notifications,
  disk temps, and platform command validation
- Update load_config.sh: detect PLATFORM (unraid/truenas/unknown), export SCRIPTS_DIR,
  auto-source Plugin/$PLATFORM/adapter.sh after common.sh
- Wire all call sites: replace direct rc.d, pgrep/pkill, var.ini, dynamix.cfg,
  disks.ini, and validate_unraid_cmd calls with platform_*() functions across
  watchdogs, orchestrators, and System_Essentials scripts
- Update all documentation: rename refs, update webgui escalation logic,
  add platform adapter section to Plugin README, update main README with
  portability vision and corrected self-healing stack description

Docker_Essentials/README-Docker_Essentials.md

@@ -153,7 +153,7 @@ its history after you've fixed the problem. No manual file editing required.
 
 ```
 Docker_Essentials/   ← acts on containers          (this folder)
-unRAID_Essentials/   ← acts on the server itself
+System_Essentials/   ← acts on the server itself
 Watchdogs/           ← reactive monitoring + last-resort stability
 Monitors/            ← observes, measures, reports
 Rsync/               ← moves data between servers