compiler/context: preserve_wildcards flag conflates reducer and vector builtin semantics for ActiveDimRef

[bpowers]

Problem

In compiler/context.rs, IndexOp::ActiveDimRef was added to has_iteration_preserving_ops to support vector builtins (VectorSortOrder, VectorElmMap, etc.) where dimension references like h[DimA] must keep their full array view. However, the same preserve_wildcards_for_iteration flag is also set by array reducers (SUM, MIN, MAX, MEAN, SIZE, STDDEV).

This means if a vector builtin is nested inside a reducer (e.g., SUM(vector_sort_order(vals[*], 1)) + SUM(z[D])), the reducer's preserve_wildcards context could incorrectly convert ActiveDimRef to Wildcard for non-array arguments.

Why it matters

• Correctness: While not currently triggered, this is a latent semantic bug that could produce wrong simulation results in specific nesting patterns.
• Maintainability: The single flag conflates two distinct compiler intentions (reducer wildcard preservation vs. vector builtin dimension reference preservation), making future changes to either feature risky.

Component(s) affected

• src/simlin-engine/src/compiler/context.rs -- has_iteration_preserving_ops and preserve_wildcards_for_iteration

Current state

Not triggered by any existing tests because:

1. Normal lowering resolves dimensions before preserve_wildcards affects them
2. lower_preserving_dimensions is only used for hoisting

Proposed fix

Introduce a separate context flag (e.g., promote_active_dim_ref) set only by vector builtins, keeping has_iteration_preserving_ops limited to Wildcard/SparseRange/Range. This cleanly separates the two concerns and prevents cross-contamination between reducer and vector builtin contexts.

Context

Identified during review of the VM vector operations branch (vm-vector-ops), specifically the addition of ActiveDimRef to iteration-preserving ops.