WebKit on iOS does not show text being composed by an IME inside the
terminal, so users type blind until it commits. Add mobile-ime-preview.js,
a visual-only controller that renders the composition in the xterm helper
layer and holds a committed chunk until local echo, parsed terminal output
or a 2s fallback shows it. Wire it into terminal-ui.js, the script order,
the build minify/hash lists and styles, with unit and wiring tests.
