refactor(output): isolate ROOT gathers on run-owned MPI lanes (ADC-683)

[wolf75222]

Summary

This draft is stacked on #646 at 836f2f28.

• duplicate one run-scoped MPI lane for ROOT scientific-output consumers
• pass only that authenticated lane into native System and AmrSystem ROOT gathers
• close the lane collectively at run shutdown and reject reuse after closure
• keep payload gathering scalable with chunked MPI_Gatherv
• fence the native bindings and runtime against falling back to WorldCommunicator or MPI_COMM_WORLD

The history intentionally remains split into three reviewable commits:

1. 5ee5cf79 — production
2. 26b9b964 — tests
3. fe1af95d — documentation

Validation

• targeted Python architecture/lifecycle matrix: 7 passed
• Ruff on modified Python and stub surfaces: passed
• serial/OpenMP syntax checks for system_fields.cpp and amr_system.cpp: passed
• MPICH/OpenMP syntax checks for the same native sources: passed
• clang-format diff: clean
• documentation checker: passed, with the pre-existing stale-docmap warning on the final specification
• git diff --check: passed

Scope and non-claims

• source-only tranche; no shared build/install was modified
• no real compiled MPI execution matrix was run in this worktree
• no custom subgroup communicator, GPU, or 3D claim
• ADC-683 remains open: core MPI/Kokkos lifecycle authorities and the Catalyst/HDF5 process-global library guards still require their own bounded audit