refactor(ui): unify terminal rendering and component primitives

[fcoury-oai]

Why

Red's dialogs, floating and docked composers, pickers, completions, panels, overlays, and plugin workspaces independently implemented prompt editing, action hints, scrolling, geometry, icons, Unicode clipping, and frame rendering. That duplication made narrow terminals, theme changes, streamed content, and document-aware highlighting inconsistent and difficult to maintain.

What changed

• Introduce concrete shared UI primitives: a real rope-backed PromptBuffer, responsive UiAction/ActionBar, selection and follow-tail viewports, half-open ScreenRect geometry, Markdown rich-text painting, and a centralized icon catalog.
• Migrate floating composers, docked panel composers, single-line input, picker selection, LSP completions, plugin workspaces, and file icons onto the appropriate shared primitive without introducing a general-purpose widget framework.
• Keep the floating composer deliberately right-aligned so code remains visible on the left; preserve multiline editing, Vim modes, grapheme-aware cursor movement, undo/redo, prompt history, and Ctrl+Enter submission.
• Recompose visible overlays on every frame and route incremental edits through the existing document-aware window renderer so overlays and multiline syntax highlighting remain correct.
• Unify themed dialog and popup frames, responsive action hints, rounded completion borders, grapheme-safe truncation, and splash-screen display-width calculations.
• Verify the plugin host API against runtime dispatch in both directions and document component ownership and intentionally separate follow-up work in docs/UI_ARCHITECTURE.md.

This is a standalone UI refactor based directly on master. It contains no parallel-agent foundation commits, agent-workspace backend, worktree or checkpoint changes, or plugin host-contract expansion.

How to Test

1. Exercise the shared components and the sensitive agent-composer regression:

   cargo test --lib ui::
   cargo test -p red --lib agent_composer_never_uses_picker_history_or_records_prompt_input
   cargo test --lib runtime_dispatch_is_covered_by_the_machine_readable_schema

   Expected: prompt history, Unicode editing, responsive actions, picker/completion behavior, right-aligned composer geometry, Ctrl+Enter submission, and bidirectional plugin-schema parity all pass.

2. Run the complete workspace and repository quality gates:

   cargo fmt --check
   cargo test --workspace --all-targets --all-features
   cargo clippy --all-targets --all-features -- -D warnings

   Expected: all workspace tests pass and formatting and Clippy report no warnings or errors.

3. Open the floating agent composer, enter a multiline Unicode draft, resize the terminal, and submit using Ctrl+Enter.

   Expected: Enter inserts a newline, the composer remains right-aligned, the editor remains visible on the left, and the exact draft is submitted without leaking into picker history.

4. Scroll upward in a streaming text panel and resize the editor while an overlay is visible.

   Expected: new transcript content does not interrupt manual scroll position, complete Unicode graphemes remain visible, and the overlay persists across subsequent rendered frames.

[fcoury]

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[chatgpt-codex-connector]

Avoid repainting the full viewport before the final frame

For ordinary edit events in EventRenderMode::Immediate, this now renders every visible row of the active window, but render_edited_window_rows neither flushes a frame nor advances render_generation. Consequently, process_editor_event still calls self.render(buffer) at src/editor.rs:7549-7552, repainting the viewport again after every inserted or deleted character. This changes the prior one-row prepaint into an extra full syntax-highlighted viewport pass and can noticeably increase input latency in large panes or expensive documents; either limit this prepaint to affected rows or make it satisfy the event's final render.

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