native: bundle arena.mjs verbatim into initramfs
Goal is parity with the web build — arena.mjs runs unmodified under
the QuickJS runtime. The docker-build.sh allowlist already had clock.mjs;
adding arena.mjs next to it. Mirror in build-and-flash-initramfs.sh.
If arena hits a missing API at runtime it should error loudly — fix by
exposing the API in js-bindings.c rather than patching arena itself, so
both targets stay in sync.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>