Preserve error references in materialize

[fallintoplace]

Summary

• forward materialize errors without copying them
• add a regression test for a non-copyable error delivered by lvalue reference

Why

materialize advertises set_error_t(error_type&) as set_value_t(set_error_t, error_type&), but its receiver took errors by value. That can require a copy and changes an lvalue into an rvalue. This forwards the error so the runtime behavior matches the completion signature.

Testing

• cmake --build build --target test.exec -j 8
• ./build/test/exec/test.exec '[materialize]'

[copy-pr-bot]

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