zephyr-cp: CIRCUITPY_USB_MSC is off, so storage USB drive control is missing #11531

Claude wrote this, with edits by @dhalbert. These are notes for whoever works on this later.

Problem

zephyr-cp presents CIRCUITPY over USB with its own MSC code in ports/zephyr-cp/supervisor/usb.c. Its build never defines CIRCUITPY_USB_MSC, so the USB parts of storage are compiled out. This causes several problems:

• disable_usb_drive(), unsafe_disable_usb_drive(), and enable_usb_drive() always raise RuntimeError: Cannot change USB devices now.
• remount() is allowed while the host has write access. It should raise "Cannot remount path when visible via USB." instead.
• remount() clears the block device lock that USB took. zephyr-cp's locked[0] stays true, so USB never takes the lock again.
• Nothing releases locked[], so the lock is not dropped on eject or unmount.

Seen on a Feather nRF52840 running zephyr-cp built from 314c0be.

What is needed

• Define CIRCUITPY_USB_MSC for zephyr-cp when USB is enabled.
• Implement LUN enabling and disabling: report CIRCUITPY not ready when storage_usb_enabled() is false.
• Provide usb_msc_release_circuitpy_lock(), usb_msc_holds_lock(), and usb_msc_remount(), as supervisor/shared/usb/usb_msc_flash.c does (the first two are added in Make storage.unsafe_disable_usb_drive() reversible #11530).

Zephyr USB MSC support

Unfortunately, some features are missing from Zephyr USB MSC that we need. Checked in adafruit/zephyr at a4d81519a1, subsys/usb/device_next/class/usbd_msc_scsi.c:

• LUN not ready: possible. There is no API for it. But TEST UNIT READY, READ, and WRITE all answer "medium not present" when the disk's status is not DISK_STATUS_OK (L425, L749, L803). zephyr-cp's _zephyr_disk_status() can return DISK_STATUS_NOMEDIA while the drive is disabled.
• Read-only to the host: not supported. MODE SENSE always reports no write protection (L600). DISK_STATUS_WR_PROTECT is treated as medium not present. So when CIRCUITPY is writable by Python, as after remount("/", readonly=False) in boot.py, the drive disappears from the host instead of appearing read-only. This needs a Zephyr change.
• Eject: no notification. START STOP UNIT is handled inside the class and does not call the disk driver (L610). zephyr-cp cannot release its lock when the host ejects the drive.