Update README_MAS.md with detailed instructions for using Apple Transporter for Mac App Store submissions

This commit enhances the `README_MAS.md` by adding comprehensive steps for installing and using the Apple Transporter app to upload `.pkg` files to App Store Connect. It includes guidance on signing in, uploading packages, and submitting builds for review, as well as optional command-line upload instructions using `iTMSTransporter`. These updates aim to streamline the submission process for developers and provide clarity on common issues encountered during uploads.

build_tools/macos/README_MAS.md

@@ -260,6 +260,46 @@ This repo’s script also supports:
 
 ## Step 7 — Upload + submit for review
 
-- Upload the `.pkg` using Apple’s **Transporter** app (App Store Connect).
-- In App Store Connect, wait for processing, select the build, then **Submit for Review**.
+### 7.1 Install Apple “Transporter” (the upload tool)
+
+Apple requires Mac App Store submissions to be uploaded using **Transporter** (a macOS app published by Apple).
+
+Where to get it:
+
+- Install **Transporter** from the **Mac App Store** (search for “Transporter”).
+  - App Store listing name is typically **“Transporter”** by Apple.
+
+### 7.2 Upload the `.pkg` with Transporter
+
+1. Open **Transporter**
+2. Sign in with the Apple ID that has access to **App Store Connect**
+3. Click **Add App** (or **+**) and choose your signed upload package:
+   - `cmake-build-macos-release-mas/strawberry-mas.pkg` (or your custom `--pkg-out` path)
+4. Click **Deliver**
+5. Wait for upload + server-side validation to complete
+
+Notes:
+
+- Uploading can take a while depending on your connection.
+- If Transporter reports an error, the message usually includes the exact App Store Connect requirement you violated (bundle id mismatch, missing entitlements, invalid signature, etc.).
+
+### 7.3 Submit the build in App Store Connect
+
+1. Open **App Store Connect** in your browser and go to **My Apps**
+2. Select your app, then go to the **macOS App** platform section
+3. Find your uploaded build under **TestFlight** or **Prepare for Submission** (Apple’s UI wording changes over time)
+4. Wait for Apple to finish “Processing” the build
+5. Select the build for your version, complete required metadata, then click **Submit for Review**
+
+### (Optional) CLI upload (advanced): `iTMSTransporter`
+
+If you prefer uploading from the command line, Apple’s underlying uploader is **iTMSTransporter**.
+On most systems it’s available via Xcode command line tools as:
+
+```bash
+xcrun iTMSTransporter -help
+```
+
+CLI upload requires additional credentials (App Store Connect API key or Apple ID auth) and is easier to get wrong than the Transporter GUI.
+For most folks, **Transporter.app is the recommended path**.