fix(firmware): halt-on-failure stopped waiting for the wave to finish (v0.24.5-beta)

A regression I introduced in v0.24.3, and described inaccurately in its own
commit message.

Adding concurrency moved the halt check to the end of a wave. I wrote that
"at the default of 1 those are the same moment". That is only true of a wave
holding one device. With several devices in one wave and the default
sequential setting, the old code stopped the moment one failed; mine went on
to attempt every remaining device in that wave first. Anyone relying on
halt-on-failure with a wide wave 1 got a wider blast radius than before,
without changing a setting.

Halting now stops further devices from *starting* as soon as a failure is
known, whatever the concurrency. Devices already in flight are allowed to
finish, because interrupting one mid-write is how you brick it. At
concurrency 1 that is precisely the previous behaviour. Above 1, only the
devices genuinely running side by side can carry a bad build -- which is the
unavoidable part of the trade in #135, rather than a cost imposed on rollouts
that never asked for concurrency.

The firmware documentation said the same wrong thing and is corrected.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -6,7 +6,7 @@
 </p>
 
 <p align="center">
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