installer: scaffold package manager abstraction and support matrix

docs/installer-support.md

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+# Installer platform support
+
+This document defines current support status for `./install.sh`.
+
+## Support states
+
+- **Stable**: full tested path in CI and expected to work end-to-end.
+- **Scaffolded**: distro is detected and actionable install guidance is provided, but full automatic package installation is not implemented yet.
+- **Unsupported**: distro/package manager is not detected by installer.
+
+## Current matrix
+
+| Distro family | Package manager | State | Notes |
+|---|---|---|---|
+| Debian / Ubuntu | `apt` | **Stable** | Full automatic dependency install path. |
+| Fedora / RHEL / CentOS | `dnf` / `yum` | **Scaffolded** | Detection + actionable commands present; auto-install path is TODO. |
+| openSUSE | `zypper` | **Scaffolded** | Detection + actionable commands present; auto-install path is TODO. |
+| Arch Linux | `pacman` | **Scaffolded** | Detection + actionable commands present; auto-install path is TODO. |
+| Other Linux distros | unknown | **Unsupported** | Installer exits with package-manager guidance requirement. |
+
+## Guard rails
+
+- Debian/Ubuntu behavior must remain stable for every portability PR.
+- New distro support should be added behind explicit package-manager adapters and tests.
+- If a distro is scaffolded but not fully automated, installer should fail fast with actionable manual commands (not generic errors).