Fix use-after-free in UMS Filesystem name/mount_name

[crux161]

Summary

Fixes a crash when accessing files from USB storage (USB key, external drive) on Switch, plus a follow-up thread-safety hardening of the UMS device callback.

Crash root cause

Filesystem::name and Filesystem::mount_name (src/fs/fs_common.hpp) are std::string_view. For USB devices, those views point into std::string members of UmsController::Device.

libusbhsfs's populate callback (UmsController::usbhsfs_populate_cb) does self->devices.clear() then re-populates on every event — partition discovery, mount, unmount. That destroys the strings the existing Filesystem views reference, leaving them dangling. Subsequent file access via mount_name.data() (e.g. through devoptab lookup) triggers a use-after-free and crashes.

Fixes in this PR

1. Own the strings (commit 1)

• Filesystem::name / mount_name now own their strings (std::string instead of std::string_view). USB Filesystem instances survive UmsController::devices.clear(). The C++23 build (gnu++23) makes the existing this->name = sv assignments in network FS subclasses still compile unchanged.
• ums_devices_changed_cb no longer calls std::erase on context.filesystems while range-iterating it (was UB). Switched to std::erase_if, and cur_fs is only reset when it actually pointed to a removed device.

2. Defer UMS updates to the main thread (commit 2)

• libusbhsfs invokes the populate callback on its USB-event thread, which previously mutated context.filesystems / cur_fs while main-thread UI code was reading them. The callback now only stashes a pending device list under a mutex; apply_pending_ums_changes drains and applies it once per frame from menu_loop and video_loop, keeping context.filesystems single-threaded for readers.
• When the filesystem hosting the currently-playing file disappears (USB unplugged mid-playback), async-stop mpv and break out of video_loop with EIO so the user falls back to the menu cleanly.

Test plan

• Built and tested on real Switch hardware.
• USB-key file access no longer crashes.
• Unplug while in the menu: device cleanly disappears from the list, currently-listed files become inaccessible with a clean error rather than a crash.
• Unplug while paused / between reads in playback: drops back to menu cleanly.
• Network filesystems and SD card unaffected (regression check).

Known remaining limitation

Physical USB removal while mpv's demuxer thread is mid-read still crashes inside libusbhsfs's internal teardown — that race lives below SwitchWave and would require upstream changes in libusbhsfs to defer cleanup until in-flight reads drain. Not addressed here.