Get Material Allocation Shares

How many units of each line EACH claiming task actually gets.

NOT the same question as /tasks/material-allocations, which totals what is CLAIMED per line for the picker's ceiling. This hands the RECEIVED units out: who is covered, who is short, and by how much.

It exists because the client cannot answer it. A planner holds one work stream's tasks, and a PO line is claimed across streams and projects — so every surface ran the allocation over the claims it could see and served the same delivery again in the next stream. Four claims totalling six units against a five-unit delivery came back as four met requirements.

Keyed "<task_id>::<item_id>", which is the key the client's own allocation map already uses, so the server's answer drops straight in where the local estimate was.