fix #12: fix terminal viewport jump during streaming

[blueberrycongee]

Summary

• Fix intermittent viewport jump to top when scrolling up during active Claude CLI output
• Root cause: BufferService.scroll() decrements buffer.ydisp by 1 for each trimmed line during buffer trimming while isUserScrolling is true, causing the viewport to drift toward ydisp=0 (stuck at top)
• Approach: Monkey-patch BufferService.scroll() to freeze buffer.ydisp via a temporary property descriptor during buffer trimming when isUserScrolling is true. The freeze blocks the trim decrement while letting onScroll events fire with the stable ydisp value, so the Viewport stays at the user's scroll position
• scrollLines() (user-initiated scrolling) is unaffected — it's a separate code path
• Prior attempts history: see docs/bugs/scroll-pinning-evolution.md

Test plan

• During Claude streaming, scroll up — viewport stays at user's position
• When not scrolling, new output auto-scrolls to bottom as normal
• No viewport lock-to-bottom side effect
• Scrolling back to bottom during streaming resumes auto-follow
• No scroll jank or flickering during rapid output