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

Monitors/README-Monitors.md

@@ -82,13 +82,13 @@ drive SMART attributes, ZFS pool state, ARC statistics, kernel memory pressure.
 ```
 Monitors/            ← observes and reports          (this folder)
 Docker_Essentials/   ← acts on containers            (docker_watchdog starts/stops)
-unRAID_Essentials/   ← acts on the server            (stability_watchdog, inotify_tuning)
+System_Essentials/   ← acts on the server            (stability_watchdog, inotify_tuning)
 Fallback/            ← acts on the full stack        (fallback, handback)
 Rsync/               ← calls bandwidth_monitor.sh    (auto-logs each sync)
 ```
 
 `weekly_health_digest.sh` reads state files written by scripts in Docker_Essentials,
-unRAID_Essentials, Fallback, and Rsync. It is the only script in this folder with
+System_Essentials, Fallback, and Rsync. It is the only script in this folder with
 runtime dependencies on other folders' output — everything else is fully independent.
 
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