add comprehensive documentation (#9537)

docs/misc/install.func/INSTALL_FUNC_INTEGRATION.md

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+# install.func Integration Guide
+
+How install.func integrates with the ProxmoxVE ecosystem and connects to other function libraries.
+
+## Component Integration
+
+### install.func in the Installation Pipeline
+
+```
+install/app-install.sh (container-side)
+    │
+    ├─ Sources: core.func (messaging)
+    ├─ Sources: error_handler.func (error handling)
+    │
+    ├─ ★ Uses: install.func ★
+    │  ├─ setting_up_container()
+    │  ├─ network_check()
+    │  ├─ update_os()
+    │  └─ motd_ssh()
+    │
+    ├─ Uses: tools.func (package installation)
+    │
+    └─ Back to install.func:
+       ├─ customize()
+       └─ cleanup_lxc()
+```
+
+### Integration with tools.func
+
+install.func and tools.func work together:
+
+```
+setting_up_container()          [install.func]
+    │
+update_os()                     [install.func]
+    │
+pkg_update()                    [tools.func]
+setup_nodejs()                  [tools.func]
+setup_mariadb()                 [tools.func]
+    │
+motd_ssh()                      [install.func]
+customize()                     [install.func]
+cleanup_lxc()                   [install.func]
+```
+
+---
+
+## Dependencies
+
+### External Dependencies
+
+- `curl`, `wget` - For downloads
+- `apt-get` or `apk` - Package management
+- `ping` - Network verification
+- `systemctl` or `rc-service` - Service management
+
+### Internal Dependencies
+
+```
+install.func uses:
+├─ core.func (for messaging and colors)
+├─ error_handler.func (for error handling)
+└─ tools.func (for package operations)
+```
+
+---
+
+## Best Practices
+
+### Always Follow This Pattern
+
+```bash
+#!/usr/bin/env bash
+source /dev/stdin <<<"$FUNCTIONS_FILE_PATH"
+
+# 1. Setup error handling
+catch_errors
+
+# 2. Initialize container
+setting_up_container
+
+# 3. Verify network
+network_check
+
+# 4. Update OS
+update_os
+
+# 5. Installation (your code)
+# ... install application ...
+
+# 6. Configure access
+motd_ssh
+
+# 7. Customize
+customize
+
+# 8. Cleanup
+cleanup_lxc
+```
+
+---
+
+**Last Updated**: December 2025
+**Maintainers**: community-scripts team