Editor: single-line / embeddable-input mode

[tig]

Summary

Terminal.Gui.TextView can be reused as a one-line (or fixed-height) input field via Multiline (false ⇒ single-line; disables word-wrap, constrains navigation), EnterKeyAddsLine (false ⇒ Enter raises Accepting instead of inserting a newline — form submit), and TabKeyAddsTab (false ⇒ Tab traverses focus instead of inserting). Editor is multi-line only.

Direction: Editor is intended to functionally replace TextView (not API/UI-compatible). A syntax-highlighted single-/few-line code or expression input (REPL line, search/expression field) is a real product want TextView serves poorly. This resolves the prior non-goal tension (specs/decisions.md OPEN-006): single-line/embeddable-input is a target, not ceded to TextView. Most of it is key-binding-level — Enter/Tab semantics + an Accepting event + a height/scroll constraint.

Not replicable by a consumer: changes core key semantics and the layout/scroll contract; not reachable via current API.

Scope

• Multiline (bool, default true) on Editor: when false, suppress newline insertion, disable WordWrap, constrain vertical nav/scroll to one row, remove Command.NewLine key binding.
• Interactions: multi-caret / vertical-multi-caret disabled (or no-op) when Multiline == false; selection still works.

Not in Scope

• EnterKeyAddsLine (default true): when false, Enter raises an Accepting event (TG Accepting/HandledEventArgs convention) instead of Command.NewLine.
• TabKeyAddsTab (default true): when false, Tab/Shift+Tab fall through to focus traversal instead of indent/insert. Users can remove the keybindgns for tab if they wan this fn.
• All three wired through commands + the configurable [ConfigurationProperty] Editor.DefaultKeyBindings where they are binding-shaped; no inline if.

Definition of done

• specs/decisions.md OPEN-006 → resolved (DEC-008): single-line/embeddable-input is a target; rationale = Editor functionally replaces TextView.
• Multiline
• Single-line mode: no newline insertion, no word-wrap, height/scroll constrained; selection + editing still work.
• specs/single-line-mode/spec.md; specs/public-api.md updated (R8).
• Failing-first tests incl. an embed-in-dialog integration test; suites green; dotnet format + jb clean.

Refs: specs/textview-parity-gap/spec.md Gap 3, specs/decisions.md OPEN-006.