test: demonstrate yaml/json parsing and generation round-trips

Add a suite covering the four supported conversions around a shared
Codable value: parsing YAML, parsing JSON (via the same API, since YAML
1.2 is a JSON superset), generating YAML, and generating strict JSON
through Foundation's JSONEncoder. Each path asserts exact output and
round-trips back to the original value.

Tests/RoundTripDemonstrationTests.swift

@@ -0,0 +1,93 @@
+import Foundation
+@testable import PureYAML
+import Testing
+
+/// End-to-end demonstrations of the four supported conversions:
+/// parsing YAML, parsing JSON, generating YAML, and generating JSON.
+///
+/// JSON parsing is handled by PureYAML directly because YAML 1.2 is a strict
+/// superset of JSON. JSON generation is delegated to Foundation, which is the
+/// correct tool for guaranteed-strict JSON output.
+@Suite("Round-Trip Demonstration")
+struct RoundTripDemonstrationTests {
+    struct Config: Codable, Equatable {
+        var name: String
+        var port: Int
+        var enabled: Bool
+        var hosts: [String]
+    }
+
+    static let config = Config(
+        name: "server",
+        port: 8080,
+        enabled: true,
+        hosts: ["a.example.com", "b.example.com"],
+    )
+
+    // MARK: Parsing YAML
+
+    @Test("Parses YAML into a typed value")
+    func test_parsesYAML() throws {
+        let yaml = """
+        name: server
+        port: 8080
+        enabled: true
+        hosts:
+          - a.example.com
+          - b.example.com
+        """
+
+        let decoded = try PureYAML.decode(Config.self, from: yaml)
+
+        #expect(decoded == Self.config)
+    }
+
+    // MARK: Parsing JSON
+
+    @Test("Parses JSON into the same typed value, since YAML is a JSON superset")
+    func test_parsesJSON() throws {
+        let json = #"""
+        {"name": "server", "port": 8080, "enabled": true, "hosts": ["a.example.com", "b.example.com"]}
+        """#
+
+        let decoded = try PureYAML.decode(Config.self, from: json)
+
+        #expect(decoded == Self.config)
+    }
+
+    // MARK: Generating YAML
+
+    @Test("Generates YAML from a typed value")
+    func test_generatesYAML() throws {
+        let yaml = try PureYAML.encodeToYAML(Self.config)
+
+        #expect(yaml == """
+        name: "server"
+        port: 8080
+        enabled: true
+        hosts:
+          - "a.example.com"
+          - "b.example.com"
+
+        """)
+
+        // The generated YAML round-trips back into the same value.
+        #expect(try PureYAML.decode(Config.self, from: yaml) == Self.config)
+    }
+
+    // MARK: Generating JSON (Foundation)
+
+    @Test("Generates strict JSON from a typed value using Foundation")
+    func test_generatesJSON() throws {
+        let encoder = JSONEncoder()
+        encoder.outputFormatting = [.sortedKeys]
+
+        let data = try encoder.encode(Self.config)
+        let json = try #require(String(bytes: data, encoding: .utf8))
+
+        #expect(json == #"{"enabled":true,"hosts":["a.example.com","b.example.com"],"name":"server","port":8080}"#)
+
+        // The Foundation-generated JSON parses back through PureYAML to the same value.
+        #expect(try PureYAML.decode(Config.self, from: json) == Self.config)
+    }
+}