feat(tui): Phase 4 — sticky prompt headers

[davidrhodus]

Continues the grok-inspired TUI work (Phases 1–3 on main). Scroll up through a long tool output and the prompt that section belongs to now pins to the top of the transcript — so you never lose track of which request you're reading output for.

╭ hi · pipe · glm-5.2 ─────────────────────────────────────╮
│❯ refactor the tokenizer                                  │   ← pinned
│  tokenizer output line 20                                │
│  tokenizer output line 21                                │
│  tokenizer output line 22                                │
╰──────────────────────────────────────────────── ↓ 41 new ╯

How

• TranscriptEntry::UserPrompt(Line) — a structurally-distinct entry (vs a plain Line) so the render pass can find prompt boundaries. flatten/text treat it like a Line; push_user_prompt records it; run.rs's two prompt echoes route through it.
• The render does one measuring pass building per-line wrapped-height prefix sums — yielding both the exact total (as before) and each prompt's wrapped-row offset. When scrolled (not following) past a prompt, the most recent prompt strictly above the viewport top is overlaid on the top inner row, with a subtle band (truecolor) marking it as pinned. No sticky while following (bottom-pinned).
• wrapped_height became a #[cfg(test)] helper (the render path sums the new per-line function inline); its overflow-saturation regression test stays.

Scope / deferred

Bounded, no render-model rewrite. Still deferred to a future pass (each needs a block cursor / live-streaming blocks / a new engine): per-block selection folding, the running accent-wave animation, mouse selection, and a real markdown/syntect engine.

Verification

153 hi-tui tests pass (new screen-dump test: pinned when scrolled past, absent while following); cargo fmt --all + full-workspace clippy -D warnings clean; ratchet updated. Ran the CI gate set locally this time (fmt --all, ratchet, full clippy, hi tests) to avoid the round-trips Phase 3 hit.

🤖 Generated with Claude Code