Issue 472 implementation notes

• Implementation is in worktree ../iris-issue-472 on branch issue-472-bg-compaction.
• Wayland now schedules automatic provider/subagent compaction as a background read-only summarizer when Tier 3 installs a worker provider factory. Harnesses without that factory keep the legacy foreground provider/excerpts path.
• The worker returns text only. The parent harness revalidates the planned durable entry-id range, derives carry/task state, registers recall, enforces shrink, appends the compaction entry, rebuilds live context, and emits the applied event.
• Stale worker results are discarded after parent revalidation. Worker errors fall back to deterministic excerpts only if the original durable range is still valid.
• The subagent summarizer setting uses the existing Wayland subagent backend with read-only tools and a bounded tool-roundtrip/output budget.
• Lifecycle events are metadata only: job id, state, covered message count, token estimate, and a short status message. Summary text and covered transcript content are not sent through lifecycle events.