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/Manual-Monitors.md

@@ -285,7 +285,7 @@ higher than 90% — at 100% utilisation new inotify watches silently fail.
 **`PHP_FPM_WARN_PCT`**: 80% means the WebGUI is using most of its workers. At 100%
 new requests queue (WebGUI feels sluggish) or time out.
 
-**`PHP_MAX_CHILDREN`**: Set by `php_fpm_max_children.sh` in `unRAID_Essentials/` — do
+**`PHP_MAX_CHILDREN`**: Set by `php_fpm_max_children.sh` in `System_Essentials/` — do
 not set manually here.
 
 ### Reading the Weekly Digest Data
@@ -297,7 +297,7 @@ shows for the week:
 
 A few warning snapshots per week is normal. A rising peak or many warnings per week
 means the limits should be adjusted — use `inotify_tuning.sh` or `php_fpm_max_children.sh`
-in `unRAID_Essentials/`.
+in `System_Essentials/`.
 
 ---
 
@@ -566,7 +566,7 @@ SILENT_MODE=false    # monitor script — output is the point
 parse_args "$@"
 
 # root check if needed
-# tool validation (validate_unraid_cmd)
+# tool validation (platform_require_cmd)
 acquire_lock
 detect_hosts    # if host-specific config needed