Add Core1 refresh recovery and shared SPI arbitration

lib/shared_spi_bus.c

@@ -0,0 +1,65 @@
+#include "shared_spi_bus.h"
+#include "pico/mutex.h"
+#include "pico/multicore.h"
+#include <stdbool.h>
+
+static mutex_t g_spi_bus_mutex;
+static bool g_spi_bus_initialized = false;
+static uint32_t g_core_depth[2] = {0, 0};
+
+void shared_spi_bus_init(void) {
+    if (g_spi_bus_initialized) {
+        return;
+    }
+
+    mutex_init(&g_spi_bus_mutex);
+    g_spi_bus_initialized = true;
+}
+
+void shared_spi_bus_lock(void) {
+    if (!g_spi_bus_initialized) {
+        shared_spi_bus_init();
+    }
+
+    int core = get_core_num();
+    if (core < 0 || core > 1) {
+        core = 0;
+    }
+
+    // Re-entrant lock on same core (needed for nested SD helpers).
+    if (g_core_depth[core] > 0) {
+        g_core_depth[core]++;
+        return;
+    }
+
+    mutex_enter_blocking(&g_spi_bus_mutex);
+    g_core_depth[core] = 1;
+}
+
+void shared_spi_bus_unlock(void) {
+    if (!g_spi_bus_initialized) {
+        return;
+    }
+
+    int core = get_core_num();
+    if (core < 0 || core > 1) {
+        core = 0;
+    }
+
+    if (g_core_depth[core] == 0) {
+        return;
+    }
+
+    g_core_depth[core]--;
+    if (g_core_depth[core] == 0) {
+        mutex_exit(&g_spi_bus_mutex);
+    }
+}
+
+void shared_spi_bus_force_recover(void) {
+    // Used after Core1 reset to avoid stale mutex/depth state.
+    mutex_init(&g_spi_bus_mutex);
+    g_core_depth[0] = 0;
+    g_core_depth[1] = 0;
+    g_spi_bus_initialized = true;
+}