fix: fix REST API and HTTP server examples

- Fix string interpolation issues: escape curly braces with \{ and \}
  since unescaped { triggers string interpolation
- Fix effectful function invocation: use "let _ = run main() with {}"
  instead of bare "main()" calls
- Use inline record types instead of type aliases (structural typing)
- Fix flake.nix to show build progress instead of suppressing output

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

flake.nix

@@ -31,7 +31,10 @@
 
           shellHook = ''
             # Build and add lux to PATH
-            cargo build --release --quiet 2>/dev/null
+            if [ ! -f target/release/lux ] || [ Cargo.toml -nt target/release/lux ] || [ src/main.rs -nt target/release/lux ]; then
+              echo "Building lux..."
+              cargo build --release
+            fi
             export PATH="$PWD/target/release:$PATH"
 
             printf "\n"