O3: positron-ratatui — terminal Renderer + Host loop (outlier A)

[joelteply]

O3 — the first real stateful Renderer + Host event loop

Validates positron's contract against a genuinely different type Output than counter-cli's String: real terminal cells (any ratatui Widget), drawn into a ratatui Buffer. This is outlier A in the outlier-validation discipline — the wgpu GPU renderer (O4) is outlier B.

What's here (positron-ratatui, three single-purpose pieces)

• render_to_buffer — project a ViewState through a Renderer into a headless Buffer. The primitive both loops and every test build on.
• TerminalHost — the Host impl: state-down via on_state, event-up via a boxed key map. Owns no terminal I/O, so the state/event contract is unit-testable without a TTY (render_current renders straight into a Buffer).
• drive (headless, generic over Backend → TestBackend in tests) + run_crossterm (thin live-TTY wrapper). Both share one dispatch step and one redraw — the state-down/event-up decision lives in exactly one place.

Define once, project many

examples/counter_tui.rs renders the same Counter ViewState the tests render headlessly — nothing about the view changes between a TestBackend assertion and a real terminal.

Tests (one mod, each with // what this catches:)

1. a Renderer whose Output is real cells (not a String) projects into a Buffer at the expected position — the whole reason this is outlier A
2. TerminalHost honors state-down (store + render) and event-up (key→command; unmapped→None)
3. drive runs the full state-down/event-up cycle headlessly via TestBackend

Notes

• ratatui 0.29 pinned in workspace deps; consumers use ratatui::crossterm (its re-export) → no separate-crossterm version-skew.
• Docs: ARCHITECTURE.md + README mark O1–O3 landed, O4 (positron-wgpu, outlier B) next.

Validated: cargo fmt --check clean, cargo clippy --all-targets -- -D warnings clean, cargo test -p positron-ratatui = 3 passed, example builds.

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