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

Tools/README-Tools.md

@@ -86,7 +86,7 @@ something new, write the tool. Store it here. Find it at 2am next time.
 ## ━━━ RELATIONSHIP TO OTHER FOLDERS ━━━
 
 ```
-unRAID_Essentials/  ← regular system maintenance    — scheduled
+System_Essentials/  ← regular system maintenance    — scheduled
 Docker_Essentials/  ← regular container management  — scheduled
 Monitors/           ← regular health reporting       — scheduled
 Orchestrators/      ← regular maintenance windows   — scheduled
@@ -103,7 +103,7 @@ Fallback/
 Docker_Essentials/
   docker_watchdog.sh ── writes skip list + history ──► watchdog_skip_list_manager.sh manages them
 
-Docker_Essentials/ + unRAID_Essentials/ + Fallback/
+Docker_Essentials/ + System_Essentials/ + Fallback/
   All continuous scripts ──────────────────────────► continuous_scripts_status.sh reads their state
 ```