Check the character in the output string instead of referring to a nonexistent grid_ member. This restores Debug compilation while preserving the intended invariant that rendering must not write an identical character twice.
Size labels intentionally replace interior spaces, so requiring an entirely unwritten destination would reject valid solutions. Update the testing notes now that Debug builds pass.
Tests: Debug and Release CTest suites (4 passed each)
Refs: #7
Exercise Results::output() with the known order-8 fixture so Debug builds compile and run the rendering assertions. Check the output dimensions and ensure the valid tiling leaves no unrendered cells.
The test passes in Release and currently fails to compile in Debug because Results::set() refers to the nonexistent grid_ member.
Refs: #7
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