* fix(wlroots): share one virtual keyboard across clients
add_client sent a keymap fd per emulation client, and it is the same fd every
time, so under client churn the queue grew past MAX_FDS_OUT and sendmsg failed
with ETOOMANYREFS. One lazily created keyboard sends it once. The pointer stays
per client because it carries no descriptor.
* test(wlroots): add a descriptor churn reproduction
Creates N emulation clients while sampling open descriptors, with a control
mode that holds the client count at one to distinguish client creation from
the event path. Motion is always zero valued so it never moves the cursor.
