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

Transcodes/ramdisk_setup.sh

@@ -7,7 +7,7 @@
 # ─────────────────────────────────────────────────────────────────────────────
 # Creates the tmpfs ramdisk, SSD fallback directory, transcode symlink, and
 # pre-creates transcoding-temp on the ramdisk. Run once at array start via
-# array_started.sh (unRAID_Essentials/). Idempotent — already-mounted ramdisk
+# array_started.sh (System_Essentials/). Idempotent — already-mounted ramdisk
 # reports status and exits cleanly. Always resets the symlink to the ramdisk
 # on boot, ensuring a clean state regardless of what state it was in before
 # shutdown.
@@ -44,7 +44,7 @@
 #   without attempting to remount or changing anything.
 #
 # Notification Validated
-#   validate_unraid_cmd confirms the notify script is present before use.
+#   platform_require_cmd confirms the notify script is present before use.
 #
 # Silent on Success
 #   Startup script runs on every boot — no output when healthy.
@@ -117,7 +117,7 @@ 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"