#21363 fix(agent-pane): preserve composer slot after first send

Summary

Keep the cx-ncc.58 composer stable through the first send and add a regression for the pending harness transition. The visible location strip disappears after the first send, but a session-scoped hidden slot preserves the bottom-anchored composer controls while transcript rows mount; queued prompts replace that slot, and reopened chats do not inherit it. The test also locks the single thinking-live row and its status transition.

This is intentionally current-base only. It does not cherry-pick the stale native strip patch or the broad motion/sidebar work from #21331.

Validation

• bun test test/composer-location-row-new-chat-only.test.tsx — 5 pass
• bun test test/transcript-live-runs.test.tsx — 4 pass
• bun run typecheck — no type errors in 1482 files
• vp fmt ... --check
• git diff --check

Hosted exact-head validation is required for agent-pane.new-chat-no-shift#send-to-streaming; the prior base receipt reproduced the 45.9px jump in both Chromium and WebKit.