perf: speed up binning by not waiting for all mapping jobs - #1061

This is performance/QoL improvement.

Right now, binners have to wait for every BOWTIE2_ASSEMBLY_ALIGN / MINIMAP2_ASSEMBLY_ALIGN to finish, because groupTuple(by:0) doesn't know how many elements will be in the tuple.

In practice this means a couple of straggler alignment jobs stall the entire shortread (or longread) binning path, even for assemblies whose mappings already completed. This inflates runtimes and makes resource usage suboptimal.

Fix: the mappings per assembly are predictable from binning_map_mode (own -> 1, group -> samples in group, all -> total samples). We compute that count and wrap the key with groupKey(key, count), so each group is released as soon as it's complete.

PR checklist

• This comment contains a description of changes (with reason).
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