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

Media/media_cleaner.sh

@@ -50,7 +50,7 @@
 #   detect_hosts()       — correct folder lists per host via MY_ID aliases
 #   Empty array guards   — warns and exits cleanly if no folders or patterns configured
 #   Folder existence     — skips missing folders with warning, continues others
-#   validate_unraid_cmd  — notify script validated before use
+#   platform_require_cmd  — notify script validated before use
 #
 # ==============================================================================================
 # CONFIGURATION
@@ -117,7 +117,7 @@ acquire_lock "wait"
 # detect_hosts() sets MY_ID and aliases HOST*_ANIME/MEDIA_CLEAN_FOLDERS
 detect_hosts
 
-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"