GPU: provide the std type_traits subset used on Metal

[ktf]

MSL has no standard library, so the handful of traits the GPU code relies on
are defined for METAL, as is already done for the other device compilers.

[davidrohr]

I don't understand this PR. It seems to just copy and paste everything from the other architectures into a separate block for Metal. Why? Is this AI generated without having a look? I think it could be simplified significantly.
And actually, I don't really know why it is needed at all, since for GPU code except for CUDA and HIP we alraedy use the ad-hoc type_trairs replacement, so it should be active for METAL already.

[ktf]

As discussed privately, this the a forward port of some old "bio" attempt where I was keeping things separate, mostly because of the enum syntax rather than constexpr bool. If you are fine with it, this is the folding of the two branches of the ifdef.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 0663dd6 at 2026-09-15 20:45:

## sw/BUILD/o2checkcode-latest/log
--
========== List of errors found ==========
++ GRERR=0
++ grep -v clang-diagnostic-error error-log.txt
++ grep ' error:'
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/TRKFT3/FT3/base/src/GeometryTGeo.cxx:111:32: error: statement should be inside braces [readability-braces-around-statements]
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/TRKFT3/FT3/base/src/GeometryTGeo.cxx:114:31: error: statement should be inside braces [readability-braces-around-statements]
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/TRKFT3/FT3/base/src/GeometryTGeo.cxx:140:43: error: statement should be inside braces [readability-braces-around-statements]
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/TRKFT3/FT3/base/src/GeometryTGeo.cxx:148:25: error: statement should be inside braces [readability-braces-around-statements]
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/TRKFT3/FT3/base/src/GeometryTGeo.cxx:152:32: error: statement should be inside braces [readability-braces-around-statements]
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/TRKFT3/FT3/base/src/GeometryTGeo.cxx:259:16: error: statement should be inside braces [readability-braces-around-statements]
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/TRKFT3/FT3/base/src/GeometryTGeo.cxx:355:52: error: statement should be inside braces [readability-braces-around-statements]
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/TRKFT3/FT3/base/src/GeometryTGeo.cxx:359:50: error: statement should be inside braces [readability-braces-around-statements]
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/TRKFT3/FT3/base/src/GeometryTGeo.cxx:365:86: error: statement should be inside braces [readability-braces-around-statements]
/sw/SOURCES/O2/slc9_x86-64-slc9_x86-64/0/Detectors/Upgrades/ALICE3/TRKFT3/FT3/base/src/GeometryTGeo.cxx:368:32: error: statement should be inside braces [readability-braces-around-statements]
++ [[ 0 == 0 ]]
++ exit 1
--

Full log here.