Remove pessimizing move from sink future return

[KjellKod]

⭐ Why this matters

Consumers compiling g3log with warnings treated as errors can currently fail on an unnecessary std::move, as reported in #564. This change restores clean compilation without changing future ownership or exception delivery.

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Summary

Return the temporary future directly from SinkHandle::call() so compilers can perform copy elision and avoid -Wpessimizing-move. Fixes #564.

Changes

• Remove the unnecessary std::move around promise.get_future() in src/g3log/sinkhandle.hpp.
• Preserve the existing std::bad_weak_ptr exception path for expired sinks.

Validation

• Configure and build all targets with the warning promoted to an error: cmake -S . -B build-issue-564 -DCMAKE_BUILD_TYPE=Release -DADD_G3LOG_UNIT_TEST=ON -DADD_G3LOG_BENCH_PERFORMANCE=ON -DPRETTY_FUNCTION=ON "-DCMAKE_CXX_FLAGS=-Wpessimizing-move -Werror=pessimizing-move" && cmake --build build-issue-564 --config Release --parallel. Expected: the complete build succeeds without a pessimizing-move diagnostic.
• Run ctest --test-dir build-issue-564 -C Release --output-on-failure. Expected: every discovered test passes; the current macOS/Linux suite reports 8/8 passing.

Watch for: test_concept_sink must continue delivering std::bad_weak_ptr through the returned future after its sink has expired.

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Quest - https://github.com/KjellKod/Quest
Co-Authored-By: Codex 
in collaboration with KjellKod