feat(editor): F4.6b the command line on a computer's desktop

A new oracle, TerminalWindowRun, types into Java's own command line
window: the window writes each line after its prompt, takes a blank
line, cls, reset and exit itself, keeps the lines for the arrow keys,
and keeps its text, history and working directory over a stop and a
start.

The room now holds a command line window per computer for everyone
(TerminalWindow), kept over stops; the protocol carries its lines and
the one way to read them (terminalScreen); the core goes on in the
working directory of the run before (continueIn). The desktop opens
programs in Java's frame, and the command line works with keyboard and
finger: grey on black, the prompt hidden while a command runs, history,
chips for the commands, Ctrl+C. The room's failure is now tested in the
browser.

Fixed: the room handed the engine the wall clock with fractions of a
millisecond, and a ping hung after its first timeout. The engine and
the room count whole milliseconds, as Java's clock does.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/protocol/src/terminal.ts

@@ -0,0 +1,82 @@
+// A command line's window as the room keeps it and every client shows it
+// (plan §7.3): its lines, in order, and how a client reads them, after
+// GUIApplicationTerminalWindow (measured with tools/java-oracle's
+// TerminalWindowRun). The window, not the program, puts the start screen
+// up, writes each entered line after its prompt, takes a blank line, cls,
+// reset and exit itself, hides the prompt while a command runs and keeps the
+// lines entered for the arrow keys.
+
+/** A line of a command line's window. */
+export type TerminalLine =
+  /** What the program or the window wrote: the start screen, a command's output, a blank line's "  \n". */
+  | { readonly kind: 'text'; readonly text: string }
+  /** A line entered after its prompt; `command` when the program runs it (not cls, reset or exit). */
+  | { readonly kind: 'input'; readonly prompt: string; readonly line: string; readonly command: boolean }
+  /** cls or reset: the window starts again (the start screen follows). */
+  | { readonly kind: 'clear' }
+  /** A command that runs on (ping …) began or ended (Java's Boolean to the window). */
+  | { readonly kind: 'busy'; readonly busy: boolean }
+  /** The prompt after a command, when it changed (the working directory). */
+  | { readonly kind: 'prompt'; readonly prompt: string }
+  /** exit: the window closes; its text stays. */
+  | { readonly kind: 'exit' };
+
+export interface TerminalScreen {
+  /** What the window shows, from its last cls on. */
+  readonly text: string;
+  readonly prompt: string;
+  /** Java hides the prompt from a command's start until its end. */
+  readonly promptShown: boolean;
+  /** Every line entered, the oldest first (blank lines not), for the arrow keys. */
+  readonly history: readonly string[];
+}
+
+/** The window as Java shows it after these lines. */
+export function terminalScreen(lines: readonly TerminalLine[]): TerminalScreen {
+  let text = '';
+  let prompt = '';
+  let promptShown = true;
+  let busy = false;
+  const history: string[] = [];
+  for (const line of lines) {
+    switch (line.kind) {
+      case 'text':
+        text += line.text;
+        if (!busy) promptShown = true;
+        break;
+      case 'input':
+        text += `\n${line.prompt}${line.line}\n`;
+        history.push(line.line);
+        if (line.command) promptShown = false;
+        break;
+      case 'clear':
+        text = '';
+        break;
+      case 'busy':
+        busy = line.busy;
+        promptShown = !busy;
+        break;
+      case 'prompt':
+        prompt = line.prompt;
+        break;
+      case 'exit':
+        break;
+    }
+  }
+  return { text, prompt, promptShown, history };
+}
+
+/**
+ * The arrow keys in the history (Java's commandHistoryPointer): -1 is the
+ * empty line below the newest, 0 the newest, and so on; up goes back, down
+ * forward, neither beyond the ends.
+ */
+export function historyStep(pointer: number, step: 'up' | 'down', length: number): number {
+  const next = pointer + (step === 'up' ? 1 : -1);
+  return Math.max(-1, Math.min(length - 1, next));
+}
+
+/** The line the history shows at a pointer: empty below the newest. */
+export function historyLine(history: readonly string[], pointer: number): string {
+  return pointer < 0 ? '' : (history[history.length - 1 - pointer] ?? '');
+}