Follow-up to salvaged PR #92767 (review round 2 P1): the .done path
allocated a fresh tail sequence even for items announced earlier via
output_item.added, so a mixed announced/pending stream without
output_index values reordered the calls ([B, A] instead of [A, B]).
First-observed ordering metadata is now recorded for every announced
item and reused at .done; a fresh sequence is allocated only for
genuinely unannounced items. The .done event's own output_index wins
when present, with the announced index as fallback.
Regressions: two announced calls without indices where the first later
receives .done; an announced non-function item preceding a pending call.
Backends that omit per-item done events on a successful completion
(anomalyco/opencode#37159) caused an announced function call to be
silently dropped: the turn ended with output == [] and the tool never
executed. Track calls announced via output_item.added, accumulate
argument deltas, and settle still-pending calls from accumulated state
at a successful terminal event. output_item.done stays authoritative.
Mirrors anomalyco/opencode#43575.
Teknium