feat: smooth streaming mode for TUI response rendering

[anandgupta42]

The TUI response rendering feels sluggish compared to Claude Code. Text appears in visible jumps and scrolling is jerky during LLM streaming.

Root cause: The <markdown> element re-lays out block elements (headers, code blocks, lists) on every token delta, causing layout height shifts that make stickyScroll jump.

Solution: Add ALTIMATE_SMOOTH_STREAMING feature flag that:

1. Uses <code filetype="markdown"> during streaming (syntax colors without layout-shifting block elements), swaps to <markdown> after completion
2. Pre-merges consecutive delta events in the SDK batch window
3. Uses direct store path updates instead of produce() proxy
4. Reduces scroll-to-bottom latency from 50ms to 0ms
5. Memoizes trim() in TextPart to avoid redundant string scans