Add COVault109 solo-cell handling

Implement solo-cell logic to suppress transform packet sends and remote proxy representation while the local player is in COVault109, allowing first-time character creation to remain visually single-player. One entry location update is sent so other clients can hold stale proxies before suppression begins. Added F4TSoloCell.h helper for load-order-independent solo cell recognition by editor ID and cached FormID.

plugin/include/F4TSoloCell.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <cstdint>
+#include <optional>
+#include <string_view>
+
+namespace F4T::SoloCell
+{
+	inline constexpr auto kVault109EditorId = "COVault109";
+
+	inline std::optional<std::uint32_t>& GetVault109CellIdCache()
+	{
+		static std::optional<std::uint32_t> s_vault109CellId;
+		return s_vault109CellId;
+	}
+
+	inline bool HasEditorId(const RE::TESObjectCELL& a_cell, std::string_view a_editorId)
+	{
+		const auto* cellEditorId = a_cell.GetFormEditorID();
+		return cellEditorId && a_editorId == cellEditorId;
+	}
+
+	inline bool IsSoloCell(const RE::TESObjectCELL& a_cell)
+	{
+		if (HasEditorId(a_cell, kVault109EditorId)) {
+			return true;
+		}
+
+		auto& vault109CellId = GetVault109CellIdCache();
+		if (!vault109CellId) {
+			const auto* vault109Cell = RE::TESForm::GetFormByEditorID<RE::TESObjectCELL>(
+				RE::BSFixedString{ kVault109EditorId });
+			if (vault109Cell) {
+				vault109CellId = vault109Cell->GetFormID();
+			}
+		}
+
+		return vault109CellId && a_cell.GetFormID() == *vault109CellId;
+	}
+}