docs polish (NUX): sandbox bwrap-install note, node 22+ prereq, testnut escape-hatch at the invoice moment

[orveth]

Three minor onboarding-doc gaps from the NUX sim (2026-08-06, rc.4):

• sandbox needs bwrap installed. seller-operate step 3 uses bwrap, which isn't present by default (bwrap: MISSING on the test box). Note it must be installed first — it's the sandbox tool for serving the open pool.
• node 22+ prerequisite. The npm-claude install row says "Node 22+", but a common fresh box (measured: debian, node v20) is below it. Flag installing/upgrading node as a prereq for the npm route (or point to the curl installer).
• no dev/testnut pointer at the invoice moment. maxplayer wallet setup prints a REAL Lightning invoice with a clear "real sats" warning, but doesn't point to the testnut/dev alternative right there — a user not ready to spend real money isn't told how to try without it at the decision point.

Full internal report: /srv/forge/share/mobee/nux-sim-20260806.md

[orveth]

Resolved by #632 (merged @7da4ea3). All three gaps listed in this issue are addressed in README.md, one-to-one:

1. sandbox bwrap install — added: bwrap (bubblewrap) is not on a stock box; install it first before serving the open pool.
2. node 22+ prereq — added: a stock box often ships Node 20; check node --version and upgrade, or use the curl installer (no Node needed).
3. testnut escape-hatch at the invoice moment — added at the wallet-setup step: a play-money maxplayer wallet setup 21 --mint https://testnut.cashudevkit.org alternative, pointed to right at the real-invoice decision point.

Delivered via the maxplayer contribution market (seller turtle 8784aec6, job b70ba078). This issue scoped exactly these three gaps; all done.