Phase 2: LittleFS storage manager on /spiffs + shared SPI2_HOST bus

main/spi_bus_manager.h

@@ -0,0 +1,27 @@
+/*
+ * MTG RFID Companion - Shared SPI bus manager
+ *
+ * Phase 2 Step 2.3: Initializes the single SPI2_HOST (VSPI) bus shared by
+ * the ST7789 display and the MFRC522 RFID reader.
+ */
+
+#pragma once
+
+#include "esp_err.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Initialize the shared SPI2_HOST bus.
+ *
+ * Must be called exactly once before the display and RFID driver attach
+ * their SPI devices. MFRC522 is passed a NULL bus_config so that it adds
+ * its device to this pre-existing bus instead of trying to own it.
+ */
+esp_err_t spi_bus_manager_init(void);
+
+#ifdef __cplusplus
+}
+#endif