perf(mpi): token protocol for the neighbour mesh and graduation exchanges

[gouarin]

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Description

Rebased on main after #476 was merged.

Every mesh construction serialized and sent the full mesh state to each neighbour three times (subdomain, cells, then the whole mesh with every derived mesh id), and every iteration of the graduation fixed point re-sent the full cell array - even when the sender's data had not changed since the previous exchange.

This PR replaces those exchanges by a token protocol: a one-int header announces whether the serialized payload follows; when it does not, the receiver keeps its cached copy of the neighbour's data. A sender only skips the payload when the receivers provably hold identical data:

• mesh exchanges (update_neighbour_subdomain, update_meshid_neighbour, update_mesh_neighbour): cells geometrically identical to the reference mesh (same_cells, from perf(mesh): detect the adaptation fixed point before constructing the new mesh #476), same neighbour set and - for the whole-mesh exchange, whose derived ids depend on the neighbours' cells - a token received from every neighbour in the cells exchange. find_neighbourhood now carries the previously exchanged neighbour meshes over to the rebuilt neighbourhood; they are the receive-side cache.
• graduation: the existing ca_changed flag is exactly "modified since the last exchange"; the neighbours' cell arrays now persist across the fixed-point iterations, so a rank at its local fixed point stops resending its identical cell array while a neighbour keeps refining.

The exchanges stay collective over the (symmetric) neighbourhood, so the communication pattern remains matched.

Measurements on advection_2d (Tf 0.1, Apple M, MPICH + libfabric, interleaved A/B vs #476):

Config	before	after
8 ranks, libfabric sockets provider	167 s	120 s (-28%)
8 ranks, FI_PROVIDER=tcp	7.6-7.7 s	7.4-7.5 s (-3%)
4 ranks	3.09-3.14 s	3.03-3.10 s (-2%)
serial	parity	parity

Tokens cut the payload volume, which pays hugely on expensive transports and grows with rank count and mesh size. On a sane transport at low rank counts, the remaining MPI cost of the adaptation loop is the number of synchronous exchange rounds - addressed in #478.

Related issue

None.

How has this been tested?

• outputs bit-identical by construction: h5diff clean vs perf(mesh): detect the adaptation fixed point before constructing the new mesh #476 in serial and at 4 and 8 ranks (demos/FiniteVolume/advection_2d.cpp, Tf 0.5);
• tests/: 350/350 serial; tests/mpi/: 24/24 at 2/3/4 ranks, including the load-balancing migration tests which exercise the carried-over neighbour caches;
• all demos build (serial and MPI).

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[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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🟢 Metrics 15 complexity · -5 duplication

Metric	Results
Complexity	15
Duplication	-5

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