Multi-output submodel with wrong number of LHS outputs leaks a cryptic MethodError

[docxology]

Summary

Multi-output submodel calls ((a, b) ~ sub(x = x)) must have the number of LHS outputs
equal to the number of missing (un-named) interfaces. When they differ, the path falls
through to MethodError: no method matching make_node!(...) with no indication of the real
problem. The single-output path already validates this and raises a clear error
(src/graph_engine.jl:1858-1873); the multi-output path (1875-1886) does not.

Root cause

prepare_interfaces_multi (src/graph_engine.jl:1881-1886) builds a NamedTuple from
missing interfaces + LHS, without checking length(missing) == length(lhs). A mismatch
leaks a MethodError.

Repro / existing test

test/multi_and_zero_output_tests.jl:76 currently pins the confusing behaviour:

@test_throws "no method matching make_node!" create_model(main_mismatch()) do model, ctx ... end

Suggested fix

Add an explicit check in prepare_interfaces (Tuple-LHS) with a clear message:

if length(missing) != n_lhs
    error("Submodel '$fform' expects $(length(missing)) output interface(s) but $(n_lhs) were provided on the LHS.")
end

Update the test to assert the readable message.