Add MPI-complete direct scalar boundary-flux integrals

[gthyagi]

Purpose

Add a collective scalar diagnostic:

total_flux = poisson.boundary_flux_integral("Top")

For integral quantities such as heat-flow/Nusselt diagnostics, recover the
integral directly from consistent nodal reactions rather than create a
pointwise flux field, recover its boundary mass, and integrate that field
again. The public solver method delegates to utilities.boundary_flux.

Mathematics and MPI

For an essential boundary whose trace basis forms a partition of unity,
summing its consistent nodal reactions gives the integrated CBF flux. Each
rank's volume assembly contains partial reactions, so all contributing
copies of a shared boundary point must participate exactly once.

The original mantle feature (6177be86) only selected points from locally
labelled facets. New tests exposed missing contributions when a rank shared
a boundary point but held no labelled boundary facet. This revision first
reduces boundary-membership flags to point owners and broadcasts them back
through the PETSc point SF, then reduces the local reaction sums. It does
not replicate global boundary coordinates or construct a flux MeshVariable.

Eight-rank exact-conduction case	Original mantle integral	Corrected integral	Exact
Triangle P1, Bottom	-2.187500	-2.500000	-2.500000
Tetrahedron P1, Bottom	-2.389206	-2.500000	-2.500000
Triangle P2, Bottom	-2.395833	-2.500000	-2.500000
Tetrahedron P2, Bottom	-2.500000	-2.500000	-2.500000

All corrected Top integrals are +2.5. Assertions use absolute tolerance
1e-8, unchanged after the MPI fix. The existing nonuniform smooth-flux
serial/parallel reference is retained as an additional check.

Contract and Limitations

• Call on all ranks after solving a continuous scalar problem, on an
  essential boundary. Vector fields are rejected before reaction assembly.
• Return the raw CBF residual sign, matching boundary_flux().
• No mean subtraction, area normalization, or Nusselt scaling. Users divide
  by the relevant area and reference conductive flux when required.
• At intersections of driven walls, shared nodal reactions mix adjacent
  contributions; this is not a facet-separated integral at those corners.
• This feature does not change existing pointwise boundary-flux recovery.

Validation

Clean branch built from development at 87091138 in an isolated install
using the existing Pixi dependencies; no other mantle changes are present.

Suite	Result	Elapsed
Full scalar boundary-flux file plus new integral tests, serial	20 passed	18.37 s
Existing parallel boundary-flux file plus new integral tests, 8 ranks	9 passed per rank	5.38 s

New exact tests cover P1/P2 triangles/tetrahedra, signed conservation,
unchanged mesh-variable registration, unknown boundaries and vector
rejection. No long coupled convection run is used as a correctness test.

@lmoresi please review the direct-reaction integral contract, especially the
point-SF membership propagation and the documented shared-corner limitation.