Add TIM::Runtime to initialize/finalize MPI and AMReX

[alperaltuntas]

As a first step toward adding TIM in protoMOMxx as a submodule, this PR moves the direct AMReX::initialize/finalize calls from protoMOMxx to TIM.

To do so, we add TIM::Runtime, an RAII encapsulation of the infrastructure runtime: constructing it brings up MPI and then AMReX (as a guest on the resulting communicator), and destroying it tears them down in reverse order. This gives the TURBO stack one owned place for initialization and shutdown of dependencies like MPI, AMReX (and, in the future, PIO).

Who owns MPI is an explicit caller decision made by choosing one of the two constructors of TIM::Runtime. This replaces the MPI_INITIALIZED runtime check FMS's mpp layer relied on.

• Owner mode — Runtime(int& argc, char**& argv): calls MPI_Init and, at destruction, MPI_Finalize (solo drivers). Aborts if MPI is already up.
• Guest mode — explicit Runtime(MPI_Comm): adopts a communicator from a caller that owns MPI (a coupler/cap) and never finalizes it. Aborts if MPI is not up.

Either way AMReX runs as a guest on that communicator (amrex::Initialize(comm)) — it never owns argc/argv or MPI. Exactly one Runtime may exist per process (AMReX's runtime state is global).

The following draft protoMOMxx PR is a preview of protoMOMxx will utilize TIM::Runtime:. Simply, MOM::Infra becomes a thin wrapper over TIM::Runtime (owner mode in solo_driver, guest mode in future NUOPC cap).
TURBO-ESM/protoMOMxx#18

Changes:

• tim/cpp/core/tim_runtime.{hpp,cpp}: the Runtime class: owner/guest constructors, comm() accessor, non-copyable/non-movable, one-per-process guard, ordered teardown (amrex::Finalize before MPI_Finalize).
• test_tim/runtime/test_runtime_owner.cpp: with an owner-mode Runtime alive, direct MPI collectives on comm() and AMReX MultiFab collective reductions work side by side.
• test_tim/runtime/test_runtime_guest.cpp: main() owns MPI like a coupler, hands MPI_COMM_WORLD to a guest-mode Runtime, runs the same coexistence checks, then verifies after Runtime's destruction that the guest did not finalize MPI and the caller's MPI is still usable.
• test_tim/CMakeLists.txt: registers both runtime test binaries (labels "unit"); the CUDA source-property handling is factored to cover them.

[Copilot]

Pull request overview

This PR introduces TIM::Runtime, a single-owner RAII runtime manager that centralizes MPI and AMReX initialization/finalization inside TIM, enabling downstream consumers (e.g., protoMOMxx) to rely on TIM for consistent startup/shutdown sequencing.

Changes:

• Added TIM::Runtime with explicit owner/guest MPI modes and ordered teardown (AMReX then MPI).
• Integrated the new runtime into the TIM C++ library build/install.
• Added unit tests validating MPI collectives and AMReX MultiFab reductions in both owner and guest modes, and updated test CMake wiring.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tim/cpp/core/tim_runtime.hpp	Declares TIM::Runtime API, owner/guest constructors, communicator accessor, and one-per-process constraints.
tim/cpp/core/tim_runtime.cpp	Implements MPI/AMReX bring-up/tear-down and the single-runtime guard.
tim/cpp/CMakeLists.txt	Adds the runtime source to tim and installs the new header.
test_tim/runtime/test_runtime_owner.cpp	Adds owner-mode runtime test (Runtime owns MPI; AMReX runs on the owned comm).
test_tim/runtime/test_runtime_guest.cpp	Adds guest-mode runtime test (caller owns MPI; Runtime must not finalize MPI).
test_tim/CMakeLists.txt	Registers the new runtime test executables and CUDA language handling for them.
.gitignore	Un-ignores tim/cpp/core/ so the new core/ subtree is tracked despite a global core ignore pattern.

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