[ITSMFT] Share framework allocation with ITSMFT

[mpuccio]

Initialize GPU and pinned-host memory pool sizes. Expose the framework device allocator through GPUChainITS and GPUO2Interface, with the ITSMFT allocator interface required by the adapter.

[alibuild]

Error while checking build/O2/fullCI_slc9 for a1d9e7a at 2026-10-06 11:07:

## sw/BUILD/O2-latest/log
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/GlobalReconstruction/reconstruction/include/ALICE3GlobalReconstruction/GPUExternalAllocator.h:33:9: error: 'void* o2::trk::GPUExternalAllocator::allocate(size_t)' marked 'override', but does not override
/sw/slc9_x86-64/GCC-Toolchain/v14.2.0-alice2-1/include/c++/14.2.0/bits/stl_construct.h:115:28: error: no matching function for call to 'construct_at(o2::trk::GPUExternalAllocator*&)'
/sw/slc9_x86-64/GCC-Toolchain/v14.2.0-alice2-1/include/c++/14.2.0/bits/stl_construct.h:96:17: error: invalid new-expression of abstract class type 'o2::trk::GPUExternalAllocator'
/sw/slc9_x86-64/GCC-Toolchain/v14.2.0-alice2-1/include/c++/14.2.0/bits/stl_construct.h:119:7: error: invalid new-expression of abstract class type 'o2::trk::GPUExternalAllocator'
ninja: build stopped: subcommand failed.

Full log here.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 7654d0e at 2026-10-06 13:45:

## sw/BUILD/O2-latest/log
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/GPU/GPUTracking/Global/GPUChainITS.cxx:19:10: fatal error: ITSMFTTracking/ExternalAllocator.h: No such file or directory
make[2]: *** [GPU/GPUTracking/CMakeFiles/GPUTracking.dir/build.make:769: GPU/GPUTracking/CMakeFiles/GPUTracking.dir/Global/GPUChainITS.cxx.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [CMakeFiles/Makefile2:777: GPU/GPUTracking/CMakeFiles/GPUTracking.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

Full log here.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 00a7430 at 2026-10-06 14:39:

## sw/BUILD/O2-latest/log
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/GPU/GPUTracking/Global/GPUChainITS.cxx:19:10: fatal error: ITSMFTTracking/ExternalAllocator.h: No such file or directory
make[2]: *** [GPU/GPUTracking/CMakeFiles/GPUTracking.dir/build.make:769: GPU/GPUTracking/CMakeFiles/GPUTracking.dir/Global/GPUChainITS.cxx.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [CMakeFiles/Makefile2:777: GPU/GPUTracking/CMakeFiles/GPUTracking.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

Full log here.

[alibuild]

Error while checking build/O2/fullCI_slc9 for b3456ca at 2026-10-06 17:16:

No log files found

Full log here.