conveyor-engine: conveyor_compile build_compiled/build_vm are a production-dead duplicate of the queue_compile pair

[bpowers]

Found by branch review of conveyor-engine before merge (the in-flight XMILE conveyor+queue implementation; specs docs/design/conveyors.md + docs/design/queues.md). Filed from a review report that exceeded its 15-finding cap.

Summary

conveyor_compile::build_compiled / build_vm (src/simlin-engine/src/conveyor_compile.rs:2124 / :2159) duplicate the unified queue_compile::build_compiled / build_vm (queue_compile.rs:819 / :1214) minus queues/couplings, and have no production callers: libsimlin's simlin_sim_new routes every conveyor+queue model through queue_compile::build_compiled. So ~21-25 conveyor tests exercise a build path production never runs, and the pair's rustdoc (claiming the split exists for "libsimlin's reset") is false.

Cost

Repo-wide grep 'conveyor_compile::build' returns only doc/string mentions plus #[cfg(test)] callers. The sole production special-stock entry is engine::queue_compile::build_compiled at src/libsimlin/src/simulation.rs:76, whose result is cached into SimState.conveyor_plans/queue_plans (simulation.rs:272-273) and is what reset recreates the VM from. queue_compile::build_compiled (819-918) already runs expand_conveyors + the Euler gate + throwaway-db compile + resolve_plans/NotSimulatable. A future change to that special-stock prelude that lands only in queue_compile's copy leaves the conveyor test suite green against the stale twin, silently forfeiting its regression value.

Evidence

• the only production build call: Some(engine::queue_compile::build_compiled(&datamodel, &model_ref.model_name)) (src/libsimlin/src/simulation.rs:76), cached at simulation.rs:272-273
• non-test grep -rn 'conveyor_compile::build' hits are all doc/string: vm.rs:723, db.rs:1067 (comment) & :1079 (error text), common.rs:439, db/diagnostic.rs:172, db/units.rs:473; the only crate::conveyor_compile::build_vm(...) call outside conveyor_compile.rs's own #[cfg(test)] module is db/ltm_unified_tests.rs:5557 (also a #[cfg(test)] file)
• byte-identical prelude: conveyor_compile.rs:2124-2153 vs queue_compile.rs:832-917 (which additionally does expand_queues/detect_coupling_specs/apply_couplings)
• false rustdoc: conveyor_compile.rs:2120-2122 claims the split exists for "libsimlin's reset, which recreates the VM from the cached compiled sim", but reset uses queue_compile's plans (queue_compile.rs:807-809)

Suggested direction

Delete the conveyor_compile build pair and repoint its ~two dozen tests at queue_compile::build_vm/build_compiled.