chore: unconnected CliIoHost logger-only implementation (#32503)

### Issue #32345

Closes #32345

### Reason for this change

Setting the ground work for our [Programmatic Toolkit](aws/aws-cdk-rfcs#654)

### Description of changes

Created an unconnected CLIIoHost with a singular initial action available `notify`. In this implementation of the soon to be defined IoHost we are only writing logs to stdout and stderr.

### Description of how you validated changes

Verified via unit testing as this is currently unconnected to the greater AWS CDK CLI

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk/lib/toolkit/cli-io-host.ts

@@ -0,0 +1,142 @@
+import * as chalk from 'chalk';
+
+/**
+ * Basic message structure for toolkit notifications.
+ * Messages are emitted by the toolkit and handled by the IoHost.
+ */
+interface IoMessage {
+  /**
+   * The time the message was emitted.
+   */
+  readonly time: Date;
+
+  /**
+   * The log level of the message.
+   */
+  readonly level: IoMessageLevel;
+
+  /**
+   * The action that triggered the message.
+   */
+  readonly action: IoAction;
+
+  /**
+   * A short code uniquely identifying message type.
+   */
+  readonly code: string;
+
+  /**
+   * The message text.
+   */
+  readonly message: string;
+
+  /**
+   * If true, the message will be written to stdout
+   * regardless of any other parameters.
+   *
+   * @default false
+   */
+  readonly forceStdout?: boolean;
+}
+
+export type IoMessageLevel = 'error' | 'warn' | 'info' | 'debug' | 'trace';
+
+export type IoAction = 'synth' | 'list' | 'deploy' | 'destroy';
+
+/**
+ * Options for the CLI IO host.
+ */
+interface CliIoHostOptions {
+  /**
+   * If true, the host will use TTY features like color.
+   */
+  useTTY?: boolean;
+
+  /**
+   * Flag representing whether the current process is running in a CI environment.
+   * If true, the host will write all messages to stdout, unless log level is 'error'.
+   *
+   * @default false
+   */
+  ci?: boolean;
+}
+
+/**
+ * A simple IO host for the CLI that writes messages to the console.
+ */
+export class CliIoHost {
+  private readonly pretty_messages: boolean;
+  private readonly ci: boolean;
+
+  constructor(options: CliIoHostOptions) {
+    this.pretty_messages = options.useTTY ?? process.stdout.isTTY ?? false;
+    this.ci = options.ci ?? false;
+  }
+
+  /**
+   * Notifies the host of a message.
+   * The caller waits until the notification completes.
+   */
+  async notify(msg: IoMessage): Promise<void> {
+    const output = this.formatMessage(msg);
+
+    const stream = this.getStream(msg.level, msg.forceStdout ?? false);
+
+    return new Promise((resolve, reject) => {
+      stream.write(output, (err) => {
+        if (err) {
+          reject(err);
+        } else {
+          resolve();
+        }
+      });
+    });
+  }
+
+  /**
+   * Determines which output stream to use based on log level and configuration.
+   */
+  private getStream(level: IoMessageLevel, forceStdout: boolean) {
+    // For legacy purposes all log streams are written to stderr by default, unless
+    // specified otherwise, by passing `forceStdout`, which is used by the `data()` logging function, or
+    // if the CDK is running in a CI environment. This is because some CI environments will immediately
+    // fail if stderr is written to. In these cases, we detect if we are in a CI environment and
+    // write all messages to stdout instead.
+    if (forceStdout) {
+      return process.stdout;
+    }
+    if (level == 'error') return process.stderr;
+    return this.ci ? process.stdout : process.stderr;
+  }
+
+  /**
+   * Formats a message for console output with optional color support
+   */
+  private formatMessage(msg: IoMessage): string {
+    // apply provided style or a default style if we're in TTY mode
+    let message_text = this.pretty_messages
+      ? styleMap[msg.level](msg.message)
+      : msg.message;
+
+    // prepend timestamp if IoMessageLevel is DEBUG or TRACE. Postpend a newline.
+    return ((msg.level === 'debug' || msg.level === 'trace')
+      ? `[${this.formatTime(msg.time)}] ${message_text}`
+      : message_text) + '\n';
+  }
+
+  /**
+   * Formats date to HH:MM:SS
+   */
+  private formatTime(d: Date): string {
+    const pad = (n: number): string => n.toString().padStart(2, '0');
+    return `${pad(d.getHours())}:${pad(d.getMinutes())}:${pad(d.getSeconds())}`;
+  }
+}
+
+export const styleMap: Record<IoMessageLevel, (str: string) => string> = {
+  error: chalk.red,
+  warn: chalk.yellow,
+  info: chalk.white,
+  debug: chalk.gray,
+  trace: chalk.gray,
+};