Introduce index-based functions for system interactions so that we can handle intersystem specific interactions

[svchb]

This PR refactors the semidiscretization and neighborhood-search internals to pass explicit system_index and neighbor_index values through pairwise update and interaction paths.
That removes repeated system_indices(...) lookups when wrapping u/v views, fetching neighborhood searches, and dispatching pairwise work.
It enables cleaner future handling of system-pair-specific interactions without changing the current behavior.

foreach_system_indexed(semi) do system_index, system
    foreach_system_indexed(semi) do neighbor_index, neighbor
        nhs = get_neighborhood_search(system, neighbor, semi,
                                      system_index, neighbor_index)
        u_system = wrap_u(u_ode, system, semi, system_index)
        u_neighbor = wrap_u(u_ode, neighbor, semi, neighbor_index)

        if is_special_pair(system, neighbor)
            interact_special_pair!(..., nhs, u_system, u_neighbor)
        else
            interact!(..., system, neighbor, semi,
                      system_index, neighbor_index)
        end
    end
end

[codecov]

Codecov Report

❌ Patch coverage is 91.58879% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.51%. Comparing base (379db88) to head (b222d5d).

Files with missing lines	Patch %	Lines
src/general/neighborhood_search.jl	70.58%	5 Missing ⚠️
src/io/io.jl	0.00%	2 Missing ⚠️
src/general/general.jl	0.00%	1 Missing ⚠️
src/general/semidiscretization.jl	97.82%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (379db88) and HEAD (b222d5d). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (379db88)	HEAD (b222d5d)
total	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1057       +/-   ##
===========================================
- Coverage   89.06%   67.51%   -21.56%     
===========================================
  Files         128      129        +1     
  Lines        9868     9917       +49     
===========================================
- Hits         8789     6695     -2094     
- Misses       1079     3222     +2143     

Flag	Coverage Δ
total	?
unit	67.51% <91.58%> (+0.32%)	⬆️

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