Make the QUIC native test build put libmsquic.so.2 where the loader needs it

The README's Linux test build has never worked: both the bridge and the
test executable carry INSTALL_RPATH "$ORIGIN" and expect libmsquic.so.2
beside them, but only scripts/build-quic-native.sh's packaged output
directory ever got that copy. A test build produced a binary that died
at once with "cannot open shared object file". Fix it where the
knowledge already lives instead of adding a README workaround: CMake
already receives MSQUIC_LIB_DIR, so the test target now copies
libmsquic.so.2 beside itself in a POST_BUILD step.

While verifying the README's own worked example end to end, found a
second, independent break in the same docker block: MSQUIC_INCLUDE_DIR
and MSQUIC_LIB_DIR were passed as paths relative to the repository
root, but CMake resolves a relative cache-variable path against the
CMakeLists.txt that consumes it, not the invocation directory, so the
compile failed on a missing msquic.h before it ever reached the
loader. Made both paths absolute with $(pwd) and documented why.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

native/GameCult.Mesh.Quic.Native/CMakeLists.txt

@@ -90,5 +90,15 @@ if(CULTMESH_QUIC_BUILD_TESTS)
     set_target_properties(cultmesh_quic_native_tests PROPERTIES
       BUILD_WITH_INSTALL_RPATH TRUE
       INSTALL_RPATH "$ORIGIN")
+    # Both binaries carry INSTALL_RPATH "$ORIGIN" and expect libmsquic.so.2
+    # beside them at load time. build-quic-native.sh's packaged output gets
+    # that copy from its own recipe; a test build has no packaging step, so
+    # the runner would otherwise die at once with "cannot open shared object
+    # file". CMake already knows where MSQUIC_LIB_DIR is, so it places the
+    # file itself instead of leaving the reader to run an extra cp by hand.
+    add_custom_command(TARGET cultmesh_quic_native_tests POST_BUILD
+      COMMAND ${CMAKE_COMMAND} -E copy
+        "${MSQUIC_LIB_DIR}/libmsquic.so.2"
+        "$<TARGET_FILE_DIR:cultmesh_quic_native_tests>/libmsquic.so.2")
   endif()
 endif()

native/GameCult.Mesh.Quic.Native/README.md

@@ -114,8 +114,8 @@ and a container that cannot ask for that cannot run them:
       scripts/build-quic-native.sh
       cmake -S native/GameCult.Mesh.Quic.Native -B artifacts/quic-native-tests/linux-x64 -G Ninja \
         -DCMAKE_BUILD_TYPE=Release -DCULTMESH_QUIC_BUILD_TESTS=ON -DCULTMESH_QUIC_DEBUG_ASSERTS=ON \
-        -DMSQUIC_INCLUDE_DIR=artifacts/dependencies/msquic-linux-2.5.9/include \
-        -DMSQUIC_LIB_DIR=artifacts/dependencies/msquic-linux-2.5.9/package/usr/lib/x86_64-linux-gnu
+        -DMSQUIC_INCLUDE_DIR=\$(pwd)/artifacts/dependencies/msquic-linux-2.5.9/include \
+        -DMSQUIC_LIB_DIR=\$(pwd)/artifacts/dependencies/msquic-linux-2.5.9/package/usr/lib/x86_64-linux-gnu
       cmake --build artifacts/quic-native-tests/linux-x64
       for scenario in closerace holdclose holdtimeout polltimeout pollbusy pollhammer zerotimeout payloadfit latecall waitseam; do
         setarch -R artifacts/quic-native-tests/linux-x64/bin/cultmesh_quic_native_tests \"\${scenario}\" || exit 1
@@ -123,7 +123,16 @@ and a container that cannot ask for that cannot run them:
     "
 
 Both lines are run from the repository root, and the mount is that root wherever
-it is: `${PWD}` in PowerShell, `$(pwd)` in a POSIX shell. `CULTMESH_QUIC_BUILD_TESTS`
+it is: `${PWD}` in PowerShell, `$(pwd)` in a POSIX shell. `MSQUIC_INCLUDE_DIR`
+and `MSQUIC_LIB_DIR` need `$(pwd)` too, even though the `cmake` invocation
+itself already runs from `/src`: CMake resolves a relative path passed as a
+cache variable against the CMakeLists.txt that consumes it
+(`native/GameCult.Mesh.Quic.Native`), not against the directory `cmake` was
+invoked from, so a bare `artifacts/...` here looks for the dependency one level
+too deep and fails the compile before it reaches the loader. The test target's
+own build now copies `libmsquic.so.2` from `MSQUIC_LIB_DIR` beside the built
+binaries, so nothing else here needs the packaged output directory `scripts/build-quic-native.sh`
+produces. `CULTMESH_QUIC_BUILD_TESTS`
 is off in the shipped configuration (see the CMakeLists.txt option comment); it
 turns on the scenario runner `tests/cultmesh_quic_native_tests.cpp` compiles into,
 and `CULTMESH_QUIC_DEBUG_ASSERTS` turns on both the bridge's internal invariant