opencode 1.18.21-lowmem.6

Reliability ports: anomalyco#43881 (retry clean-EOF empty provider streams — a flaky gateway can no longer end a turn silently; adds a regression test), anomalyco#42176 (mark finish: "error" on stream failures), anomalyco#43607 (SSE heartbeat comments no longer reset the chunk timeout). PORTS.md gains the durability/rebase policy and upstream watch-list. Recommended round.