claude feed: show a missing-span marker when the proxy transport reports a gap #1381

Follow-up of #1376 (steering q87).

What happens: claude-code-headless#64 reports proxy-events generations deleted before the app read them (transport-gap, lostGenerations). As of #1376:

• ClaudeSession re-emits it as proxy-transport-gap;
• SessionManager records a claude.proxy_transport_gap incident (always on, carried in debug bundles) and re-emits proxy-transport-gap with the session id.

Nothing consumes that re-emit. The live semantic feed and the transcript show the surviving chunks as if they were continuous, and the user sees no missing span. It happens only when the main process's poller stalls through at least ~1 GiB of proxy traffic.

Fix shape:

• Route proxy-transport-gap to the renderer (session IPC) and render a bounded marker at that point in the feed (for example "Some live output was not captured").
• Close or seal any flow left open by the gap.
• Pin the path from the manager event to the rendered marker.

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