task 3.1-3.2: IAssembler interface + StubAssembler emit primitives

Add IAssembler seam (Assemble(text, origin)) with StubAssembler default
backend. StubAssembler provides EmitU8/EmitU32/EmitU64 little-endian
byte emitters (zero dep, no FASM). Assemble throws NotSupportedException
on StubAssembler (text assembly deferred to IcedAssembler, Phase 8).

7 new tests: EmitU8, EmitU32 x2, EmitU64 x2, IS-A check,
Assemble throws. All passing (total: 64).

WhiteMagicTest/StubAssemblerTests.cs

@@ -0,0 +1,78 @@
+using WhiteMagic.Assembly;
+
+namespace WhiteMagicTest;
+
+/// <summary>
+/// Tests for <see cref="StubAssembler"/> emit primitives (<see cref="StubAssembler.EmitU8"/>,
+/// <see cref="StubAssembler.EmitU32"/>, <see cref="StubAssembler.EmitU64"/>) and the
+/// <see cref="IAssembler"/> interface contract.
+/// </summary>
+public class StubAssemblerTests
+{
+    private static StubAssembler Create() => new();
+
+    // ── Emit primitives ────────────────────────────────────────────────────
+
+    [Fact]
+    public void EmitU8_appends_a_single_byte()
+    {
+        var sut = Create();
+        var buffer = new List<byte>();
+        sut.EmitU8(buffer, 0xAB);
+        Assert.Equal([0xAB], buffer);
+    }
+
+    [Fact]
+    public void EmitU32_appends_little_endian()
+    {
+        var sut = Create();
+        var buffer = new List<byte>();
+        sut.EmitU32(buffer, 0x11223344);
+        Assert.Equal([0x44, 0x33, 0x22, 0x11], buffer);
+    }
+
+    [Fact]
+    public void EmitU32_appends_zero()
+    {
+        var sut = Create();
+        var buffer = new List<byte>();
+        sut.EmitU32(buffer, 0);
+        Assert.Equal([0x00, 0x00, 0x00, 0x00], buffer);
+    }
+
+    [Fact]
+    public void EmitU64_appends_little_endian()
+    {
+        var sut = Create();
+        var buffer = new List<byte>();
+        sut.EmitU64(buffer, 0x1122334455667788);
+        byte[] expected = [0x88, 0x77, 0x66, 0x55, 0x44, 0x33, 0x22, 0x11];
+        Assert.Equal(expected, buffer);
+    }
+
+    [Fact]
+    public void EmitU64_appends_high_bits()
+    {
+        var sut = Create();
+        var buffer = new List<byte>();
+        sut.EmitU64(buffer, 0xDEADBEEF_CAFEBABE);
+        byte[] expected = [0xBE, 0xBA, 0xFE, 0xCA, 0xEF, 0xBE, 0xAD, 0xDE];
+        Assert.Equal(expected, buffer);
+    }
+
+    // ── IAssembler interface ───────────────────────────────────────────────
+
+    [Fact]
+    public void StubAssembler_is_an_IAssembler()
+    {
+        var sut = Create();
+        Assert.IsAssignableFrom<IAssembler>(sut);
+    }
+
+    [Fact]
+    public void Assemble_from_StubAssembler_throws_NotSupported()
+    {
+        var sut = Create();
+        Assert.Throws<NotSupportedException>(() => sut.Assemble("nop", 0));
+    }
+}