fix: move doctest to buildInputs to enable aarch64 cross-compilation

[Copilot]

nix build .#packages.x86_64-linux.smart-piano-engine-aarch64 fails because CMake cannot find the doctest package during cross-compilation configure.

Root cause

doctest was in nativeBuildInputs, which in a cross-compilation context means it's only available to host-side tools. CMake's find_package(doctest REQUIRED) searches CMAKE_PREFIX_PATH for the target sysroot, where doctest was absent.

Change

In engine.nix, move doctest from nativeBuildInputs to buildInputs:

nativeBuildInputs = [ cmake cppcheck llvm ninja pkg-config ];
buildInputs = [ doctest rtmidi ] ++ lib.optionals stdenv.isLinux [ alsa-lib libjack2 ];

doctest is a C++ header library linked into test executables compiled for the target — it belongs in buildInputs for both native and cross builds.

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[github-actions]

📊 Coverage Report

Coverage Summary

Filename                      Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
include/GameEngine.hpp              4                 0   100.00%           2                 0   100.00%           7                 0   100.00%           0                 0         -
include/IGameMode.hpp               3                 0   100.00%           2                 0   100.00%           2                 0   100.00%           0                 0         -
include/IMidiInput.hpp              1                 0   100.00%           1                 0   100.00%           1                 0   100.00%           0                 0         -
include/ITransport.hpp              1                 0   100.00%           1                 0   100.00%           1                 0   100.00%           0                 0         -
include/Logger.hpp                 27                 2    92.59%           7                 0   100.00%          46                 1    97.83%          16                 2    87.50%
include/Message.hpp                11                 0   100.00%           5                 0   100.00%          10                 0   100.00%           2                 0   100.00%
include/Note.hpp                    8                 0   100.00%           2                 0   100.00%           7                 0   100.00%           4                 0   100.00%
include/RtMidiInput.hpp             4                 0   100.00%           4                 0   100.00%           7                 0   100.00%           0                 0         -
include/UdsTransport.hpp            4                 0   100.00%           3                 0   100.00%           8                 0   100.00%           0                 0         -
src/AnswerValidator.cpp            67                 7    89.55%           5                 0   100.00%          99                 7    92.93%          48                12    75.00%
src/ChallengeFactory.cpp           19                 0   100.00%           5                 0   100.00%          58                 0   100.00%          10                 0   100.00%
src/ChordGame.cpp                  53                 0   100.00%           4                 0   100.00%         104                 0   100.00%          40                 1    97.50%
src/GameEngine.cpp                 64                 2    96.88%           7                 0   100.00%         128                 3    97.66%          42                 4    90.48%
src/NoteGame.cpp                   39                 1    97.44%           4                 0   100.00%          76                 0   100.00%          28                 3    89.29%
src/RtMidiInput.cpp                56                 7    87.50%          16                 6    62.50%         136                16    88.24%          32                 7    78.12%
src/UdsTransport.cpp               82                 3    96.34%           8                 0   100.00%         126                12    90.48%          51                 6    88.24%
src/main.cpp                       25                 2    92.00%           3                 0   100.00%          52                10    80.77%          14                 6    57.14%
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                             468                24    94.87%          79                 6    92.41%         868                49    94.35%         287                41    85.71%