feat: editor/ sample app — multi-buffer text editor (Stage 4 §3.2)

[rorygraves]

Summary

Closes another §3.2 sample-app gap. A focused multi-buffer editor that
exercises `widgets.MultiLineInput` + `widgets.SplitPane` +
`widgets.MenuBar` in one app.

Layout

• Menu bar (File / Edit) at the top.
• `SplitPane` — buffer list on the left, editor on the right.
• `StatusBar` at the bottom showing buffer count, dirty state, and the most recent action.

Buffers

• Starts with one untitled buffer.
• `Ctrl+N` adds a new untitled buffer.
• `Ctrl+O` opens a `Dialogs.textInput` overlay; Enter loads the path into a new buffer; Esc cancels.
• `Ctrl+S` writes the current buffer (refuses untitled — surfaced in the status bar).
• `Ctrl+W` closes the active buffer; closing the only buffer leaves a fresh untitled.
• `Ctrl+I` (Ctrl-Tab on most terminals) switches to the next buffer.

Quit

• `Ctrl+Q` / `Ctrl+C` exits when no buffers are dirty.
• With dirty buffers, surfaces a `Dialogs.confirm` overlay; Tab flips Yes/No focus, Enter commits, Esc cancels.
• `q` is a printable character inside the editor — not a quit key.

Menu

• `F2` moves focus to the menu bar; arrows + Enter operate it via `MenuBar.handleKey`. The picked `(menuIdx, itemIdx)` routes through `applyMenuPick` to the same buffer ops as the keyboard shortcuts. `Esc` returns focus to the editor.

Tests

15 specs covering:

• initial state
• Ctrl+N / Ctrl+W / Ctrl+I buffer ops
• dirty tracking via printable characters
• Enter-in-editor produces a newline (not a quit)
• `Ctrl+O` flow against a real temp dir on disk: type path → Enter loads file
• `Ctrl+S` round-trip writes to disk and clears the dirty flag
• `Ctrl+S` on an untitled buffer reports the error and keeps dirty
• Ctrl+Q with no dirty exits via the App
• Ctrl+Q with dirty surfaces ConfirmQuit; Tab flips focus; Enter commits
• F2 ↔ Esc for menu focus
• Menu File → New round-trips through `MenuBar.handleKey`

Test plan

• `sbt termflowSample/testOnly termflow.apps.editor.EditorAppSpec` — 15/15 pass
• `sbt ciCheck` — green
• `mdbook build` — clean
• CI green on this PR
• Manual smoke: `sbt editorDemo` in a real terminal

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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