test: no fixture for a queue backlog or a belt shared with a queue #954

Two queue and conveyor behaviours have no model file in test/conveyors/ or test/queues/:

• a belt with both a queue-fed inflow and an ordinary equation-driven inflow, where the queue's volume uses up capacity the equation inflow then competes for;
• a queue that builds a backlog of several batches, served under each of the four one_at_a_time / batch_integrity combinations.

queue_coupled_conveyor.xmile has neither: its belt has only the queue inflow, and its queue has an <overflow/>, so it never holds a second batch.
docs/design/queues.md section 12 asks for a backlog fixture, test/queues/queue_wait.xmile, which was never added.

What already covers it

• Shared capacity: conv_vol_charges_capacity_room in src/simlin-engine/src/wasmgen/belt_tests.rs (VM against wasm). It uses a conveyor-fed inflow, not a queue-fed one.
• Batch rules with a backlog: every_coupled_batch_rule_matches_vm and both_batch_integrity_takes_are_inclusive_at_the_exact_budget in the same file, and VM expected values in coupling_one_at_a_time_takes_at_most_front_batch and coupling_batch_integrity_never_splits_the_front_batch in src/simlin-engine/src/queue_compile.rs.
• XMILE round trip of the batch attributes: conveyor_block_all_fields_roundtrip in src/simlin-engine/src/xmile/variables.rs.

Fix

If still wanted, add test/queues/queue_wait.xmile (no <overflow/>, arrivals faster than the belt takes them) and wire it into QUEUE_CORPUS_FIXTURES in tests/integration/simulate.rs.

Pointers

• test/conveyors/README.md, "Known corpus gaps"
• test: no conveyor fixture file for arrest, sample and other options #949 (the same kind of gap for single conveyors)