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>

docs/OVERVIEW.md

@@ -128,10 +128,16 @@ let content = File.read("config.txt")
 File.write("output.txt", "Hello!")
 let exists = File.exists("file.lux")
 
-// HTTP effects
+// HTTP client effects
 let response = Http.get("https://api.example.com/data")
 Http.post("https://api.example.com/submit", "{\"key\": \"value\"}")
 
+// HTTP server effects
+HttpServer.listen(8080)
+let req = HttpServer.accept()  // Returns { method, path, body, headers }
+HttpServer.respond(200, "Hello, World!")
+HttpServer.stop()
+
 // Random effects
 let n = Random.int(1, 100)
 let coin = Random.bool()