feat(tui): add TUI foundation with event architecture and ratatui components

[hakula139]

Summary

Add a TUI foundation with ratatui + crossterm, including an event architecture, component system, and agent loop refactoring. This is the structural plumbing that future work (markdown rendering, multi-line input, tool display, viewport virtualization) builds on.

The core design decision is the AgentSink trait, which decouples the agent loop from display. The same agent_turn code drives all three modes — the sink implementation determines where events go. ChannelSink sends events through an mpsc channel for the TUI's tokio::select! event loop; StdioSink writes directly to stdout / stderr for the bare REPL and headless modes. This keeps the agent loop DRY across all display modes.

• Add tui/event.rs: AgentEvent enum (stream tokens, thinking, tool calls, turn complete, error), UserAction enum, AgentSink trait with ChannelSink and StdioSink implementations. tool_call_title helper extracts display titles (bash commands shown inline) for both sinks.
• Add tui/app.rs: root App struct with tokio::select! event loop multiplexing crossterm events, agent channel events, and a 60 FPS tick interval for render coalescing. Layout: status bar (top) + chat (fill) + input (bottom). finish_turn helper consolidates the turn-complete reset sequence. App creates a single Theme and passes it to all component constructors.
• Add tui/component.rs: Component trait (handle_event + render) and Action enum for upward communication from components to the root app.
• Add tui/components/chat.rs: scrollable chat message list with streaming buffer. Auto-scrolls on new content, pauses on manual scroll up, resumes at bottom. Cell for content height avoids double build_text per frame.
• Add tui/components/input.rs: single-line input with cursor navigation, character insertion, backspace, forward-delete. Enter submits, Ctrl+C / Ctrl+D quits. Cached char_count avoids repeated O(n) scans; byte_offset helper DRYs up char-to-byte index conversion.
• Add tui/components/status.rs: status bar displaying ox │ model │ status with pipe separators. Status: Idle (green), Streaming (yellow), ToolRunning (yellow).
• Add tui/terminal.rs: terminal init / restore, synchronized output (BeginSynchronizedUpdate / EndSynchronizedUpdate for flicker prevention), panic-safe restore hook.
• Add tui/theme.rs: Catppuccin Mocha palette with 11 named color slots (grouped: text hierarchy → surfaces → semantic accents → status indicators by ascending severity) and style helpers. Copy derive for zero-cost pass-by-value. separator_span helper DRYs up the styled pipe separator. Transparent background, designed for future user-configurable overrides.
• Refactor main.rs into three entry points: run_tui (default), bare_repl (--no-tui), headless (-p). Tool registry created once and passed to all modes. agent_turn / stream_response take &dyn AgentSink. apply_delta returns Result to propagate sink errors. Abort agent task on quit to prevent hanging on active API streams.
• Add docs/research/tui.md: reference project analysis (claude-code Ink architecture, opencode, Codex, Rust TUI apps), flickering root cause investigation, streaming markdown strategy, design decisions.

Changes

File	Description
Cargo.toml	Add ratatui, crossterm, futures to workspace dependencies
crates/oxide-code/Cargo.toml	Wire workspace deps to binary crate
client/anthropic.rs	#[derive(Clone)] on Client, model() accessor, preserve initial text from ContentBlockInfo::Text
main.rs	Three entry points (run_tui, bare_repl, headless), tool registry created once and passed, agent_turn / stream_response take &dyn AgentSink, apply_delta returns Result, abort agent task on quit
tui.rs	Module root with pub(crate) submodules
tui/event.rs	AgentEvent, UserAction, AgentSink trait, ChannelSink, StdioSink, tool_call_title helper
tui/theme.rs	Catppuccin Mocha palette (11 color slots, Copy), style helpers, separator_span
tui/terminal.rs	Terminal init / restore, synchronized output, panic hook
tui/component.rs	Component trait and Action enum
tui/components/chat.rs	Scrollable chat with streaming buffer, auto-scroll, Cell content height
tui/components/input.rs	Single-line input with cursor, Delete key, cached char_count, byte_offset helper
tui/components/status.rs	Status bar with model and status indicator
tui/app.rs	Root App with tokio::select! loop, shared Theme, finish_turn, tool_call_title
CLAUDE.md	Updated crate structure diagram with full tui/ module tree
docs/roadmap.md	Move TUI foundation to "Working Today", update "Current Focus"
docs/research/tui.md	TUI research: reference projects, flickering, ecosystem, design decisions
docs/research/README.md	Research docs index

Test plan

• cargo fmt --all --check — clean
• cargo build compiles cleanly
• cargo clippy --all-targets -- -D warnings — zero warnings
• cargo test — 226 tests pass
• ox launches TUI with status bar, chat area, input area
• ox --no-tui launches bare REPL with same agent behavior
• ox -p "hello" runs headless and exits
• Typing visible in input, Enter submits, response streams in chat
• Mouse scroll and keyboard scroll (when input disabled) work
• Ctrl+C / Ctrl+D quit from any state
• Quit during active streaming aborts cleanly (no hang)
• pub(crate) visibility on all TUI types