PiTerminal didn't call term.focus() on mount, so xterm.js stayed
passive — onData only fires when the DOM element has focus. Users
saw the boot prompt but their keystrokes went to whatever element
held focus when they clicked Run (canvas, code editor), never
reaching the bridge. Calling focus() right after fit() makes the
prompt receive input the moment it's visible.
The qemu_manager change adds INFO-level logging when serial_input
WebSocket messages reach send_serial_bytes — useful diagnostic for
future Pi3 input problems (proves whether bytes reached the backend
before we look at TTY / kernel / PL011 wiring).
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