Constant RETAINED_TEMPLATE_BUDGET_BYTES 

pub(super) const RETAINED_TEMPLATE_BUDGET_BYTES: usize = _; // 268_435_456usize

Ceiling on the template geometry a cross-batch stream retains (#5407).

Sized against what it buys, not what it costs a small model: the representations that repeat across batches are the model’s distinct instanced shapes, whose geometry is what the shared stream sends ONCE anyway. 256 MiB of raw template buffers is several times that on every fixture measured for #5407, while staying well under one parse’s working set on the models a streaming deployment exists for.