A checkout-sized tar sits silent for a minute plus, which reads like a
hang. bsdtar (macOS) and GNU tar disagree on progress options, so poll
the growing archive and overwrite the line every 2s; the loop doubles
as a liveness signal and the final wait still propagates tar's exit.
The backup root is typically the same internal disk, so the size saving
buys nothing; measured on an M1 over a 3.3G checkout, gzip made the
backup 5x slower (74s vs 14s). Store hermes-home.tar and
electron-userdata.tar uncompressed.
Hand-off kit for proving an existing source install can move to a
branch through the real update surfaces: pre (backup + arm a
transport-level insteadOf redirect at a serve.git of the target ref),
then 'hermes update', then post (rollback + restore-exactness report).
PLAN.md holds the design; smoke-test.* is the maintainer self-check.
