conveyor-engine: queue_compile duplicates ~9 conveyor_compile helpers verbatim despite a shared pub(crate) seam

[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

queue_compile.rs re-implements conveyor_compile helper-for-helper instead of sharing through the already-established pub(crate) seam it imports 8 items from. Byte-identical (or identical bar an ErrorCode/noun) copies:

• canon (queue_compile.rs:70 vs conveyor_compile.rs:341)
• placeholder_zero_equation (:197 vs :1479)
• equation_dims (:207 vs :1490)
• equation_scalar_strings (:218 vs :937; its own doc admits "Mirrors the private helper")
• the driven-flow-read rejection scan (:426-453 vs :768-795)
• the container rewrite/attach/apply loops (:469-526 vs :817-902)
• resolve_plans' per-element flattening core with byte-identical n_queues/n_belts and eoff closures (:571-628 vs :1559-1658)
• the emit_queue_ltm_degraded_warnings twin (db/diagnostic.rs:248 vs :194)

Cost

~9 hand-maintained copies across two sibling stock types. A corrective edit to one copy silently skips its sibling: teaching the driven-read scan a new equation-reference position, or preserving Arrayed shape in placeholder_zero_equation, fixes conveyors but leaves queues (or vice versa) unpatched -- and for the scan and placeholder that is silent mis-simulation of the untouched stock type, the exact loud-rejection contract both modules document. A third discrete stock type would triple every copy.

Evidence

• the seam already exists: use crate::conveyor_compile::{ContainerMeta, ContainerNaming, ContainerPlan, ContainerVarSpec, container_value_from_slice, element_subscripts_for_dims, make_container_stock, rewrite_container_equation}; (queue_compile.rs:64-67)
• placeholder_zero_equation queue:198-203 == conveyor:1480-1485
• equation_scalar_strings doc at queue:218 says "Mirrors the private helper of the same name in crate::conveyor_compile"; body queue:219-235 == conveyor:938-954
• driven-read scan queue:433-450 == conveyor:775-792, differing only in ErrorCode::QueueDrivenFlowRead vs ConveyorDrivenFlowRead
• resolve_plans eoff queue:583-589 == conveyor:1580-1586; n_queues queue:555-557 == n_belts conveyor:1542-1544
• db/diagnostic.rs:198-204 (conveyor) vs :252-258 (queue): identical scan/sort/one-Warning-per-name, differing only in ErrorCode::{Conveyor,Queue}LtmDegraded + message noun

Suggested direction

Hoist these into conveyor_compile (or a shared discrete-stock module) exactly as the 8 already-shared items were, parameterizing the scan/warning on ErrorCode + noun.