feat: merge completed distributed dynamic filters

Retain every latest producer report, including incomplete updates, and count each task completion exactly once. Most producers wait until all planned tasks report completion; collect-left hash joins narrowly use the first complete replica.

    task 0: incomplete F0 ---> retained
    task 0: complete F0 -----+
    task 1: complete F1 -----+--> flat protobuf OR --> merged F

    collect-left task 0: complete F0 --> merged F immediately

Author: jayshrivastava