fix(renderer): flush queued view before insert above

[yanmxa]

Summary

• flush the latest queued inline View before insertAbove calculates scroll geometry
• propagate a flush failure instead of inserting scrollback against stale renderer state
• add a regression test that verifies the current frame is rendered before the committed block

Root cause

render only queues the latest View; the frame ticker flushes it asynchronously. Println can therefore reach insertAbove after the model has produced a shorter view but before that view has reached the terminal.

At that point, insertAbove uses the previous cellbuf and cursor geometry. Rows from the stale managed frame can then be moved into native scrollback and remain there permanently.

Flushing at the renderer boundary keeps the queued view, cellbuf, terminal output, and the subsequent scroll calculation on the same frame.

Fixes #1736.

Downstream regression and integration coverage: genai-io/san#401

Testing

• go test ./...
• go test -race ./...