test: round-trip the full scalar type matrix through Codable containers

Enumerate every scalar Codable type (Int8 through Int64, Int, UInt
through UInt64, Float, Double, Bool, String) through both a keyed
container, via an all-scalars struct, and an unkeyed container, via
per-type arrays. This drives the keyed encode and decode container
methods and the encoder's scalar ladder to near-full coverage and pins
exact round-trip equality for each type.

Tests/CodableTypeMatrixTests.swift

@@ -0,0 +1,60 @@
+@testable import PureYAML
+import Testing
+
+@Suite("Codable Type Matrix")
+struct CodableTypeMatrixTests {
+    struct AllScalars: Codable, Equatable {
+        var int8: Int8
+        var int16: Int16
+        var int32: Int32
+        var int64: Int64
+        var intValue: Int
+        var uintValue: UInt
+        var uint8: UInt8
+        var uint16: UInt16
+        var uint32: UInt32
+        var uint64: UInt64
+        var floatValue: Float
+        var doubleValue: Double
+        var boolValue: Bool
+        var stringValue: String
+    }
+
+    @Test("Every scalar type round-trips through a keyed container")
+    func test_keyedContainerRoundTripsEveryScalarType() throws {
+        let value = AllScalars(
+            int8: -8, int16: -16, int32: -32, int64: -64, intValue: -1,
+            uintValue: 1, uint8: 8, uint16: 16, uint32: 32, uint64: 64,
+            floatValue: 1.5, doubleValue: 2.5, boolValue: true, stringValue: "text",
+        )
+        let encoded = try PureYAML.encode(value)
+        #expect(try PureYAML.decode(AllScalars.self, from: encoded) == value)
+    }
+
+    @Test("Every scalar type round-trips through an unkeyed container")
+    func test_unkeyedContainerRoundTripsEveryScalarType() throws {
+        try expectArrayRoundTrip([Int8(-8), 8])
+        try expectArrayRoundTrip([Int16(-16), 16])
+        try expectArrayRoundTrip([Int32(-32), 32])
+        try expectArrayRoundTrip([Int64(-64), 64])
+        try expectArrayRoundTrip([Int(-1), 1])
+        try expectArrayRoundTrip([UInt(1), 2])
+        try expectArrayRoundTrip([UInt8(8), 9])
+        try expectArrayRoundTrip([UInt16(16), 17])
+        try expectArrayRoundTrip([UInt32(32), 33])
+        try expectArrayRoundTrip([UInt64(64), 65])
+        try expectArrayRoundTrip([Float(1.5), 2.5])
+        try expectArrayRoundTrip([Double(3.5), 4.5])
+        try expectArrayRoundTrip([true, false])
+        try expectArrayRoundTrip(["a", "b"])
+    }
+
+    private func expectArrayRoundTrip<Element: Codable & Equatable>(
+        _ array: [Element],
+        sourceLocation: SourceLocation = #_sourceLocation,
+    ) throws {
+        let encoded = try PureYAML.encode(array)
+        let decoded = try PureYAML.decode([Element].self, from: encoded)
+        #expect(decoded == array, sourceLocation: sourceLocation)
+    }
+}