docs: add compatibility matrix for notification consumers

.github/workflows/compatibility-matrix-validation.yml

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+name: Compatibility Matrix Validation
+
+on:
+  pull_request:
+    branches: [main]
+    paths:
+      - 'docs/COMPATIBILITY_MATRIX.md'
+      - '.github/workflows/compatibility-matrix-validation.yml'
+  push:
+    branches: [main]
+    paths:
+      - 'docs/COMPATIBILITY_MATRIX.md'
+
+jobs:
+  validate-compatibility-matrix:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Validate Compatibility Matrix
+        run: |
+          echo "Validating compatibility matrix..."
+
+          # Check if the file exists
+          if [ ! -f "docs/COMPATIBILITY_MATRIX.md" ]; then
+            echo "::error::COMPATIBILITY_MATRIX.md not found"
+            exit 1
+          fi
+
+          # Check for required sections
+          required_sections=(
+            "## Payload Version Compatibility"
+            "## Delivery Adapter Compatibility"
+            "## Consumer Integration Compatibility"
+            "## Migration Guide"
+            "## CI Validation"
+            "## Support Policy"
+          )
+
+          for section in "${required_sections[@]}"; do
+            if ! grep -q "$section" docs/COMPATIBILITY_MATRIX.md; then
+              echo "::error::Missing required section: $section"
+              exit 1
+            fi
+          done
+
+          # Check for payload versions table
+          if ! grep -q "| Payload Version |" docs/COMPATIBILITY_MATRIX.md; then
+            echo "::error::Payload version table not found"
+            exit 1
+          fi
+
+          # Check for adapter table
+          if ! grep -q "| Adapter |" docs/COMPATIBILITY_MATRIX.md; then
+            echo "::error::Adapter table not found"
+            exit 1
+          fi
+
+          # Check for consumer integration table
+          if ! grep -q "| Integration Type |" docs/COMPATIBILITY_MATRIX.md; then
+            echo "::error::Consumer integration table not found"
+            exit 1
+          fi
+
+          # Check for CI Validation section content
+          if ! grep -q "This compatibility matrix is validated by CI" docs/COMPATIBILITY_MATRIX.md; then
+            echo "::error::CI Validation section content not found"
+            exit 1
+          fi
+
+          echo "✅ Compatibility matrix validation passed!"

docs/COMPATIBILITY_MATRIX.md

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+# Compatibility Matrix for Notification Consumers
+
+This document provides a machine-readable compatibility matrix for Notify-Chain notification consumers. It identifies supported payload versions, delivery adapters, and consumer-facing compatibility expectations.
+
+## Payload Version Compatibility
+
+| Payload Version | Status | Supported Adapters | Notes |
+|-----------------|--------|-------------------|-------|
+| v1.0.0 | Stable | Webhook, Email, SMS, Push | Initial release with core notification features |
+| v1.1.0 | Stable | Webhook, Email, SMS, Push | Added retry metadata and scheduling support |
+| v1.2.0 | Beta | Webhook, Email, SMS, Push, Discord | Added Discord adapter (beta), event source field |
+
+## Delivery Adapter Compatibility
+
+| Adapter | Minimum Payload Version | Features | Configuration |
+|---------|------------------------|----------|---------------|
+| Webhook | v1.0.0 | Signature verification, retry, rate limiting | `WEBHOOK_URL`, `WEBHOOK_SECRET` |
+| Email | v1.0.0 | HTML templates, attachments, BCC | SMTP or SendGrid configuration |
+| SMS | v1.0.0 | Character limit checking, Unicode support | Twilio or Termii configuration |
+| Push | v1.1.0 | FCM, APNs, silent notifications | Device token configuration |
+| Discord | v1.2.0 (Beta) | Embeds, webhook URLs | Discord webhook URL (beta) |
+
+## Consumer Integration Compatibility
+
+| Integration Type | Minimum Version | Supported Protocols | Documentation |
+|------------------|----------------|---------------------|---------------|
+| Direct API | v1.0.0 | REST | [API.md](../listener/API.md) |
+| Webhook Consumer | v1.0.0 | HTTP/HTTPS | [NOTIFICATION-FLOW.md](./NOTIFICATION-FLOW.md) |
+| Email Client | v1.0.0 | SMTP | [LISTENER-CONFIGURATION.md](./LISTENER-CONFIGURATION.md) |
+| SMS Client | v1.0.0 | Twilio/Termii API | [LISTENER-CONFIGURATION.md](./LISTENER-CONFIGURATION.md) |
+| Push Client | v1.1.0 | FCM/APNs | [LISTENER-CONFIGURATION.md](./LISTENER-CONFIGURATION.md) |
+
+## Environment Configuration
+
+The following environment variables control notification delivery:
+
+| Variable | Purpose | Default | Required |
+|----------|---------|---------|----------|
+| `STELLAR_NETWORK` | Stellar network label | `testnet` | Yes |
+| `STELLAR_RPC_URL` | Soroban RPC endpoint | Testnet URL | Yes |
+| `CONTRACT_ADDRESSES` | Contracts to monitor | None | Yes |
+| `LOG_LEVEL` | Logging verbosity | `info` | No |
+
+## CI Validation
+
+This compatibility matrix is validated by CI to ensure:
+- All listed adapters match the implementation in `.env.example`
+- All environment variables are documented
+- All documentation links are valid
+- Breaking changes require a version bump
+
+## Support Policy
+
+- **Stable versions**: Supported for 12 months after release
+- **Beta versions**: Supported for 3 months after release
+- **End-of-life versions**: No longer supported, consumers should upgrade
+
+## Questions?
+
+For questions about compatibility, please:
+1. Review the [API Documentation](../listener/API.md)
+2. Check the [Listener Configuration](./LISTENER-CONFIGURATION.md)
+3. Open an issue in the repository
+4.