docs: Add branch version marker for 9.18 LTS support

- Identify 9.18 branch as BIND9 9.18.x LTS support line
- Document supported platforms and backporting policies
- Link to version support policy and documentation

.bind9version

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+# BIND9 9.18 LTS Support Branch
+
+Branch: 9.18
+Purpose: Long-term support for BIND9 9.18.x (ISC LTS release)
+Status: Active
+Support Level: Full
+
+## Version Information
+
+- **BIND9 Version**: 9.18.x (LTS)
+- **Supported Until**: Aligned with ISC BIND9 9.18 LTS support timeline
+- **Branch Base**: BIND9 9.18.44 (as of February 2026)
+
+## Supported Platforms
+
+- Debian 11 (Bullseye)
+- Debian 12 (Bookworm)
+- Debian 13 (Trixie)
+- Ubuntu 20.04 LTS
+- Ubuntu 22.04 LTS
+- Ubuntu 24.04 LTS
+
+## Policies
+
+### Backporting
+- ✅ Security fixes: Always backported
+- ✅ Bug fixes: Backported if compatible
+- ❌ New features: Case-by-case (no breaking changes)
+- ❌ Breaking changes: Never
+
+### Release Numbering
+Uses MAJOR.MINOR.PATCH (e.g., v1.2.3)
+
+## Key Documentation
+
+- [Version Support Policy](docs/VERSION_SUPPORT.md)
+- [Configuration Grammar](CONFIGURATION_GRAMMAR.md)
+- [Changelog](CHANGELOG.md)
+- [README](README.md)
+
+## Next Major Version
+
+For BIND9 9.20+ support, see the `main` branch which will eventually have version-specific releases.