The blob is [init] followed by per-segment events that interleave all tracks' chunks, so a single track's segments are intentionally NOT contiguous (the other track's chunk sits between them). Assert the correct invariant instead: all chunks across all tracks, ordered by offset, tile the blob with no gaps from len(header) to the blob's end. Verified in the streamplace-2 cgo container: TestFinalizeHeaderConcat and TestRegenerateSidecars both pass. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>