Change implicit rep to natural output type

[chiphogg]

From now on, whenever we do not name the repin a conversion, we'll
obtain whatever type we would naturally get if we wrote out the
conversion by hand. The main places this makes a difference are
promotable integers, and expression templates such as Eigen.

To get the mechanics right, we need to make a bunch of updates to our
:conversion_strategy target. Many of these involve explicitly
templating intermediate result types, instead of assuming they're the
same as the input type. But the main new interface property is that an
output rep of void is our signal for omitting the final cast
operation.

Then, we take advantage of this by changing our implicit-rep conversion
functions in Quantity to use void as the rep type. (QuantityPoint
follows naturally because it uses Quantity under the hood.)

We also need to tweak our overflow risk check in in_impl. We need to
make the integer promotion carve-out explicit.

One common forced downstream change is that our conversion risk checkers
need to change to their explicit-rep versions in explicit-rep contexts
that we didn't formerly recognize as such. The main place this crops up
is in Constant, where we had formerly been manifesting the constant as
T{1}, and then just using implicit rep to convert to the target unit.

Finally, as prep for the vector/matrix changes, I realized our ordering
in the main PermitAsCarveOutForIntegerPromotion implementation was not
as efficient as it could be. I decided to move the is_integral checks
closer to the top, as they should be a lot more common than promotion
and unsigned checks, which don't really even make sense for non-integral
types.

Helps #484. This PR is a major unblocker for Eigen being usable and
performant.

[Copilot]

Pull request overview

This PR changes “implicit rep” conversions (i.e., .in(unit) / .as(unit) without an explicit rep) to return the natural C++ result type you’d get by writing the conversion expression manually (notably impacting integer promotion and expression-template types like Eigen). It accomplishes this by treating an output rep of void as the signal to omit the final cast in the conversion operation pipeline.

Changes:

• Extend :conversion_strategy so conversions can model intermediate/output types correctly, and interpret NewRep = void as “no final cast; return natural output type”.
• Update Quantity implicit-rep conversion paths to use void output rep and adjust overflow-risk logic to explicitly account for integer-promotion carve-outs.
• Update Constant to run overflow/truncation checking and conversions in explicit-rep mode where appropriate; refresh and add tests to reflect the new semantics.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
fuzz/quantity_runtime_conversion_checkers_test.cc	Updates fuzz test to use explicit-rep lossiness checking where needed.
au/quantity.hh	Switches implicit .in()/.as() to void output rep, adjusts overflow carve-out handling, and updates runtime conversion checker helpers.
au/quantity_test.cc	Adds/updates tests for integer promotion behavior and distinguishes implicit vs explicit rep overflow checking.
au/conversion_strategy.hh	Implements NewRep = void behavior and corrects intermediate typing for rational conversions.
au/conversion_policy.hh	Reorders integer-promotion carve-out checks and adds a safe specialization for Rep = void.
au/constant.hh	Ensures Constant::as uses explicit-rep runtime checks and explicit-rep conversion when producing a T.
au/constant_test.cc	Adds tests validating explicit rep output types and overflow checking behavior for Constant.

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