engine: audit and clean up #[allow(dead_code)] annotations

[bpowers]

After the interpreter removal (#420), several #[allow(dead_code)] annotations remain in src/simlin-engine/src/. Some are correct (fields set by production struct literals but only read in tests), but others may be masking genuinely dead code that should be deleted or gated with #[cfg(test)].

A cleanup pass should:

1. grep -rn '#\[allow(dead_code)\]' src/simlin-engine/src/ to enumerate all annotations
2. For each, determine whether the item is:
   • Production code that clippy can't trace (e.g. salsa dispatch, struct fields set via literals) -- keep #[allow(dead_code)] with a comment explaining why
   • Test-only code -- change to #[cfg(test)] so it's excluded from the production binary
   • Genuinely dead -- delete it
3. Remove any #[allow(dead_code)] that was added defensively during the interpreter removal without verifying the item's actual usage

Known items from #420 review:

• compiler/mod.rs: Module struct fields runlist_initials, runlist_order, module_refs -- set by production code but only read in tests
• compiler/expr.rs: SubscriptIndex::strip_loc(), Expr::strip_loc()
• ast/expr2.rs: IndexExpr2::get_var_loc(), Expr2::get_var_loc() -- called via salsa dispatch
• ast/mod.rs: Ast<Expr2>::get_var_loc() -- called via salsa dispatch
• ast/array_view.rs: ArrayView::is_contiguous()