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/virtual-hosting.toml

@@ -0,0 +1,35 @@
+# Pollux Virtual Hosting Example Configuration
+#
+# This example shows how to configure multiple Gemini capsules
+# on a single server instance.
+
+# Global settings (applied to all hosts unless overridden)
+bind_host = "0.0.0.0"
+port = 1965
+log_level = "info"
+max_concurrent_requests = 1000
+
+# Main website
+["example.com"]
+root = "/var/gemini/example.com"
+cert = "/etc/ssl/example.com.crt"
+key = "/etc/ssl/example.com.key"
+
+# Blog subdomain
+["blog.example.com"]
+root = "/var/gemini/blog"
+cert = "/etc/ssl/blog.example.com.crt"
+key = "/etc/ssl/blog.example.com.key"
+
+# Personal site
+["tilde.example.com"]
+root = "/home/user/public_gemini"
+cert = "/etc/ssl/tilde.crt"
+key = "/etc/ssl/tilde.key"
+
+# Development site (different port)
+["dev.example.com"]
+root = "/home/dev/gemini"
+cert = "/etc/ssl/dev.crt"
+key = "/etc/ssl/dev.key"
+port = 1966