test: indexed (compound) assignment evaluation order and single-evaluation

[msooseth]

Adds a semantic test pinning down two invariants for indexed assignment:

• In a[f()] = g(), the RHS g() is evaluated before the LHS index f(), so RHS side effects are observable when the index is computed.
• In a[f()] += v, the LHS index f() is evaluated exactly once; the slot computation is not re-evaluated for the write side.

Coverage gap

No existing semantic test exercises a side-effecting function call as the LHS index of a compound assignment. Closest neighbors and why they don't cover it:

• viaYul/tuple_evaluation_order.sol — tuple-LHS order; not indexed, not compound.
• operators/compound_assign{,_transient_storage}.sol — compound assign on plain state vars; no side-effecting LHS.
• modifiers/evaluation_order.sol, operators/userDefined/operator_evaluation_order.sol, errors/require_error_evaluation_order_*.sol — evaluation order in modifiers / user-defined operators / require args; different mechanism.
• cleanup/cleanup_in_compound_assign.sol — truncation cleanup, not order or single-eval.
• variables/mapping_local_compound_assignment.sol — LHS-with-side-effect, but plain =.
• array/indexAccess/bytes_index_access.sol, libraries/internal_call_attached_with_parentheses.sol — indexed compound assign, but constant index and no side effects.

Complements viaYul/tuple_evaluation_order.sol.