Interface DecodedInstancedShard

Whether the stride reaches trailing field 2, the finish (#5984). Optional so a shard built by hand before it existed still type-checks; the decoder always sets it.

Whether the declared instance stride reaches the trailing itemId field. REQUIRED rather than optional on purpose: it is what a consumer keys the per-occurrence itemId allocation off (prepareInstancedRender), and an omitted flag would read as "no ids" and silently drop them — the absence- looks-like-success shape this whole change is about. The encoder derives it from the data, so false means no occurrence in the shard names an item, not merely that this build cannot see them.