docs(app-boot): correct the pre-fix capture claim and pin the exit seam contract

The PTY capture does continue past the terminal-takeover bytes with the
fatal diagnostic; only the reset never follows. State that precisely in
both notes.

Document on FailLoudProcess.exit that callers treat it as the end of the
run, matching how the release path already relies on it.

packages/ui/app-boot/src/index.ts

@@ -295,6 +295,11 @@ export interface FailLoudProcess {
   on(event: 'unhandledRejection', handler: (err: unknown) => void): unknown
   off(event: 'unhandledRejection', handler: (err: unknown) => void): unknown
   stderr: { write(chunk: string): unknown }
+  /**
+   * Terminate the process. Callers treat this as the end of the run, as
+   * `process.exit` is; a fake that returns lets the caller continue, which only
+   * a test observes.
+   */
   exit(code: number): void
 }