Adds an env-var lever for Phase 3.2's use_quadlet_backends flag so the
20× harness can flip the path on per-node without a config.json edit
(which would require an archipelago.service restart — and that triggers
FM3 cgroup cascade until Phase 3.5 ships, so we can't ask anyone to
reconfigure live nodes that way today).
Truthy parsing centralised in `parse_truthy_env` (1, true, yes, on —
case-insensitive, whitespace-trimmed). Anything else is false. The
helper is unit-tested so future env-var flags can reuse the same shape.
Also adds a default-off regression test for use_quadlet_backends so
flipping the default ahead of the 20× verification fires immediately.
TESTING.md documents the Environment= snippet for the systemd drop-in
so the next operator can flip the flag on a debug node without
re-deriving the recipe.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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