engine(vm): scalar SUM(VECTOR ELM MAP(...)) fails VM compilation #367

Problem

A scalar reduction over an array-producing builtin fails VM compilation:

result = SUM(VECTOR ELM MAP(source[*], offsets[*]))

Interpreter evaluation succeeds, but VM compilation fails with:

Cannot push view for expression type Discriminant(7) - expected array expression

Repro

Use a simple model with:

• source[D] = [10,20,30]
• offsets[D] = [2,0,1]
• result = SUM(VECTOR ELM MAP(source[*], offsets[*]))

Then run VM compilation (Simulation::compile() path).

Expected

VM should compile and produce result = 60 (matching interpreter).

Actual

VM compilation errors before simulation starts.

Notes

Discovered while working on issue #364 (nested array-producing builtins in A2A context). This appears to be a separate VM/codegen limitation for scalar reductions over array-producing builtin calls.