feat: Implement virtual hosting for multi-domain Gemini server

- Add hostname-based request routing for multiple capsules per server
- Parse virtual host configs from TOML sections ([hostname])
- Implement per-host certificate and content isolation
- Add comprehensive virtual host testing and validation
- Update docs and examples for multi-host deployments

This enables Pollux to serve multiple Gemini domains from one instance,
providing the foundation for multi-tenant Gemini hosting.

examples/single-host.toml

@@ -0,0 +1,16 @@
+# Pollux Single Host Example Configuration
+#
+# Example configuration for a single Gemini capsule.
+# For multiple hosts, use virtual hosting instead.
+
+# Global settings
+bind_host = "127.0.0.1"
+port = 1965
+log_level = "info"
+max_concurrent_requests = 100
+
+# Host configuration
+["example.com"]
+root = "./content"
+cert = "./tmp/cert.pem"
+key = "./tmp/key.pem"