Qiskit / coheriq / 41090332625
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Explain why only one engine can be active per domain (#28)

* Explain why only one engine can be active per domain

The documentation asserted the one-engine-per-domain rule in both the overview
and the design FAQ without ever giving a reason for it.  The FAQ's non-goals
section said what to do instead -- compose engines through inheritance -- but
not why the restriction is drawn there.

The reason is about whose problem compatibility becomes.  Deciding which
engines work together is genuinely hard: two engines may replace an overlapping
set of candidates, assume incompatible data representations, or expect
exclusive use of a device.  The party least equipped to decide is the user, who
typically wrote neither engine, and permitting simultaneous activation would
hand them that question by default every time they install a second one.
Permitting a single engine means it never arises, because whichever engine is
active was designed as a consistent whole by one author.

The new entry is paired with the escape hatch, since the restriction governs
what activation does rather than what is achievable.  A user who wants
overrides from two engines can build one that inherits from both, and the
compatibility question is then answered explicitly, by someone who chose to
take it on, in a named engine that can be tested as a unit.  Nothing stops that
engine from being a bad idea; the point is that it has to be asked for, and the
result is one coherent implementation rather than an ambiguous runtime blend.

The overview's bare bullet gets the short form of the reason and points at its
composition section, which gains a label for the purpose.  The FAQ non-goal
entry links to the new rationale rather than restating it.

Assisted-by: Claude Opus 5

* Tighten the one-engine rationale prose

Cuts the announcement of the answer ahead of the answer itself, merges two
sentences that carried one idea, and drops the closing reassurance that nothing
stops a combined engine fro... (continued)

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