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/emby_database_repair.sh

@@ -41,7 +41,7 @@
 #   Emby can take time to stop cleanly — 30s is intentionally generous.
 #
 # Tool Validation
-#   validate_unraid_cmd confirms sqlite3 and the notify script are present
+#   platform_require_cmd confirms sqlite3 and the notify script are present
 #   before use. jq is checked separately — required for config path detection.
 #
 # Post-Restart Verify
@@ -96,12 +96,12 @@ if [[ "$EUID" -ne 0 ]]; then
 fi
 
 # Validate required tools
-validate_unraid_cmd \
+platform_require_cmd \
     "$(command -v sqlite3 2>/dev/null || echo /usr/bin/sqlite3)" \
     "--version" "." \
     "sqlite3" || { error "sqlite3 not found — install sqlite package"; exit 1; }
 
-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"