Ephemeral Reasoning Window

Done2026-10-08Jean Mertz

Tracking ticket for RFD 120.

Implementation plan

Rename StderrRows to WindowRows: Renames StderrRows in jp_config::style::stderr_rows to WindowRows, since its name and docs describe child processes. No behavior or config change; the existing stderr_rows keys and serialized values stay the same. Mergeable on its own.
Add reasoning window config: Adds ReasoningWindowConfig under style.reasoning.window with rows (default off) and timer (default true), including the usual config trait implementations and user-facing doc comments. The display doc comment points at it. No renderer effect yet. Mergeable on its own; depends on phase 1's WindowRows.
Render reasoning into the status-region window: Adds Printer::window_rows, the single emission seam, per-block placement, window lifetime and release, region background, chrome-width wrapping, timer bookkeeping parity, and the live-flow gate. Covered by renderer tests in render/chat_tests.rs, a screen-level test via jp_pty, and a manual terminal check for flicker, default size, resize, and the post-interrupt window. Depends on phase 2.