TranscriptContainer gated committed-prefix compaction on version === undefined, so version-tracked AssistantMessageComponent history never compacted and every stream tick re-walked all N sealed blocks (depth-linear compose).
Compact fully-committed finalized blocks regardless of post-finalize version tracking: their rows are immutable native scrollback the terminal owns. A post-commit mutation no longer recommits on ordinary frames (no duplication) and rehydrates on the next destructive full replay (no loss). Adds the permanent bench/transcript-compose.bench.ts fixture; ratio(N5000/N500) drops 2.30 -> 0.90.
Fixes#5930
roboomp