shared/filesystem: mount SD at filesystem_init for reliable MSC probe

The lazy automount in tud_msc_test_unit_ready_cb can lose races with
macOS's USB MSC probe timing -- the host asks whether LUN 1 is ready
before the card has finished mounting, sees NOT_READY, and may give up
before trying again. Mounting during filesystem_init ensures the SD
card is live by the time USB enumerates, so the first probe succeeds.

Affects boards that define DEFAULT_SD_CARD_DETECT; behavior on boards
without an auto-mount path is unchanged.

Author: mikeysklar

supervisor/shared/filesystem.c

@@ -219,6 +219,13 @@ bool filesystem_init(bool create_allowed, bool force_create) {
 
     #if CIRCUITPY_SDCARDIO
     sdcardio_init();
+    #if defined(DEFAULT_SD_CARD_DETECT) && CIRCUITPY_SDCARD_USB
+    // Mount the SD card now so it's ready when USB enumerates.
+    // Lazy mount from tud_msc_test_unit_ready_cb can lose races with
+    // macOS's probe timing. Gated on CIRCUITPY_SDCARD_USB to match the
+    // existing call site in usb_msc_flash.c (guarded by SDCARD_LUN).
+    automount_sd_card();
+    #endif
     #endif
 
     return true;