perf(tui): add the reviewed adaptive stream pacer

[akougkas]

Summary

• add terminal.smoothStreaming with off, auto, and on modes while keeping off as the exact current coalescer behavior
• pace only canonical derived visible text and thinking through one ordered generation queue
• make boundary flushing and final-frame settlement awaitable, grapheme-safe, deadline-aware, and backpressure-aware
• conservatively bypass pacing for non-TTY, accessibility, low-motion, unsafe terminal, and observed-backpressure cases
• document config and environment precedence and add deterministic clock, ordering, abort/retry, folded-thinking, resize/scroll, final-frame, slow-output, and PTY contracts

Stack

Depends on the corrected frame instrumentation in the foundations PR. The instant-shell work is intentionally excluded and follows in a separate PR.

Validation

The complete stack passes typecheck, lint, build, 4,241 core tests, coverage, ci, ci:release, corrected PTY harnesses, and supported Node 22/24 runs.