feat: add schema evolution type system integration and HTTP server effect

Schema Evolution:
- Preserve version info in type resolution (Type::Versioned)
- Track versioned type declarations in typechecker
- Detect version mismatches at compile time (@v1 vs @v2 errors)
- Support @v2+ (at least) and @latest version constraints
- Store migrations for future auto-migration support
- Fix let bindings to preserve declared type annotations

HTTP Server Effect:
- Add HttpServer effect with listen, accept, respond, respondWithHeaders, stop
- Implement blocking request handling via tiny_http
- Request record includes method, path, body, headers
- Add http_server.lux example with routing via pattern matching
- Add type-checking test for HttpServer effect

Tests: 222 passing (up from 217)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/types.rs

@@ -1074,6 +1074,55 @@ impl TypeEnv {
             },
         );
 
+        // Add HttpServer effect
+        let http_request_type = Type::Record(vec![
+            ("method".to_string(), Type::String),
+            ("path".to_string(), Type::String),
+            ("body".to_string(), Type::String),
+            ("headers".to_string(), Type::List(Box::new(Type::Tuple(vec![Type::String, Type::String])))),
+        ]);
+        env.effects.insert(
+            "HttpServer".to_string(),
+            EffectDef {
+                name: "HttpServer".to_string(),
+                type_params: Vec::new(),
+                operations: vec![
+                    EffectOpDef {
+                        name: "listen".to_string(),
+                        params: vec![("port".to_string(), Type::Int)],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "accept".to_string(),
+                        params: vec![],
+                        return_type: http_request_type.clone(),
+                    },
+                    EffectOpDef {
+                        name: "respond".to_string(),
+                        params: vec![
+                            ("status".to_string(), Type::Int),
+                            ("body".to_string(), Type::String),
+                        ],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "respondWithHeaders".to_string(),
+                        params: vec![
+                            ("status".to_string(), Type::Int),
+                            ("body".to_string(), Type::String),
+                            ("headers".to_string(), Type::List(Box::new(Type::Tuple(vec![Type::String, Type::String])))),
+                        ],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "stop".to_string(),
+                        params: vec![],
+                        return_type: Type::Unit,
+                    },
+                ],
+            },
+        );
+
         // Add Some and Ok, Err constructors
         // Some : fn(a) -> Option<a>
         let a = Type::var();