Responses ingress streaming and bounded parsing boundary

Chose incremental ingress parsing only for /v1/responses and /v1/codex/responses over universal streaming because only their shared item normalizer can consume input incrementally. Below 256 KiB, preserve exact JSON.parse; above it, replay once and stream $.input.* through createInputItemsBuilder(). This bounds Lore gateway parsing amplification, not upstream allocation: if OpenCode first constructs and serializes the full request body, downstream streaming cannot recover that memory. Audit OpenCode’s construction and serialization boundary before porting the builder.