Rich Live: bounded tail-window for long responses (no stale frames)

[chauncygu]

Replaces the plain-streaming fallback for long responses with a bounded tail window so live Markdown rendering is preserved at any length without the duplicate/stale frames some terminals leave behind.

ui/render.py:

• Keep the Live region bounded to the viewport. A short response is shown in full; once it would overflow, render the whole response and feed Live only the last limit rendered lines (a tail window that always fits), so the cursor never has to redraw content that scrolled into the scrollback.
• flush_response() commits the complete output once (full frame frozen in place; tail window cleared then re-printed in full), including on Ctrl-C.
• Plain streaming kept only as a safety net (precise render failed, or the terminal is too small to bound a window).
• _live_update() bails out if Live was disabled concurrently instead of dereferencing None (defensive).
• Fast-path gate widened to 3x headroom so the cheap line estimate can't underestimate Markdown expansion (tables, block spacing) into an overflow.

tests/test_render_streaming.py: rewritten for the tail-window design — full-frame streaming, full->tail transition, tail-window commit-on-flush, real Segments rendering, and both safety-net fallbacks.

Docs: update README/news/features/advanced and the 7 i18n READMEs to
describe the bounded tail window (was: plain-streaming fallback).