Compute a real noexcept for the sum, pack and choice surface

[Bronek]

The dispatch machinery promised noexcept unconditionally while invoking user callbacks and constructing user types, so a throwing callback or a throwing copy called std::terminate - and since the specification also read noexcept == true, callers could not even detect the hazard by asking. The constructors were wrong the other way: carrying no specifier at all, they under-promised, so sum<int>{42} was noexcept(false).

is_nothrow_invocable / is_nothrow_invocable_r (stubbed false until now) are the foundation, and are computed by asking the invoke chain itself, which grows the specification it was missing. That composes: a pack answers for the call over its elements, a sum for the call over every alternative - one throwing alternative makes the whole dispatch throwing, since which one runs is a run-time choice. The recursion terminates because each step strictly reduces the number of pack/sum operands.

On top of that: the constructors weigh what they construct and relocate, append weighs the element built and every element moved into the new pack, operator== weighs the alternatives' own comparisons, and choice stops over-promising where optional and expected never did.

Two traps worth naming. A noexcept-specifier is an ordinary constant expression, not a requires-clause: both operands of its || must be well-formed, so "this alternative is not compared, and need not be comparable" has to be a guarded trait rather than a disjunction. And an explicit noexcept on a defaulted copy/move that disagrees with the implicit specification deletes it - choice's had to drop theirs once sum's became conditional.

Closes #45
Closes #280

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Stack: P6 of 13 — the release-0.1 bugfix queue, merging bottom-up into main. Based on #299 and to be retargeted to main once it merges; the diff shows only this PR's commits. Every stack boundary was validated with a gcc build and the full test suite on top of main, and the aggregate branch ran the full CI matrix green (#289).

[augmentcode]

🤖 Augment PR Summary

Summary: This PR makes the public noexcept surface for sum, pack, and choice match real behavior, avoiding both over-promising (terminate on throw) and under-promising (missing nothrow guarantees).

Changes:

• Propagates conditional noexcept through the internal invoke chain and typelist dispatch helpers
• Implements is_nothrow_invocable/is_nothrow_invocable_r by querying the invoke chain itself (instead of a stubbed false)
• Adds a brace-init-aware _nothrow_initializable trait and applies it to sum/choice constructors and related operations
• Refines pack::append, pack::invoke(_r), and relocation accounting to reflect real throwing behavior
• Updates/extends tests to pin the new noexcept contracts across monads and dispatch paths

Technical Notes: The nothrow computation composes across pack/sum dispatch (runtime choice means “all alternatives must be nothrow”). It also accounts for brace-vs-paren initialization differences and guarded traits where noexcept operands must remain well-formed.

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[augmentcode]

Review completed. 2 suggestions posted.

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[Bronek]

augment review

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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✅ All modified and coverable lines are covered by tests.

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[augmentcode]

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