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

Orchestrators/intermediate_sync_maintenance.sh

@@ -70,7 +70,7 @@ if [[ "$EUID" -ne 0 ]]; then
     exit 1
 fi
 
-validate_unraid_cmd \
+platform_require_cmd \
     "/usr/local/emhttp/plugins/dynamix/scripts/notify" \
     "" "" \
     "unRAID notify script" || warn "unRAID notify script not found — native notifications disabled"