check_platform accepted Termux as plain Linux, so `curl | bash` on a
phone walked the source-install ladder: a glibc uv, a lock whose CPython
is the bundled bionic build with no Android wheels, and on-device sdist
builds. The signed APT package is the only supported Termux shape, so
detect Termux the way the runtime does (TERMUX_VERSION or the com.termux
PREFIX) and stop before any stage with `pkg install hermes-agent` and
the setup docs URL. --json surfaces the reason in the stage frame.
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