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).

WhiteMagic/Assembly/IAssembler.cs

@@ -0,0 +1,17 @@
+namespace WhiteMagic.Assembly;
+
+/// <summary>
+/// Abstraction over an x86/x64 assembler. The default <see cref="StubAssembler"/>
+/// hand-emits calling-convention trampolines (no parsing, zero dep). An optional
+/// <see cref="IcedAssembler"/> (Phase 8) handles arbitrary mnemonics via the Iced
+/// library.
+/// </summary>
+public interface IAssembler
+{
+    /// <summary>
+    /// Assembles text mnemonics into machine code.
+    /// </summary>
+    /// <param name="assemblyText">The assembly text (Intel syntax).</param>
+    /// <param name="origin">The base address for relative encodings.</param>
+    byte[] Assemble(string assemblyText, ulong origin = 0);
+}

WhiteMagic/Assembly/StubAssembler.cs

@@ -0,0 +1,49 @@
+namespace WhiteMagic.Assembly;
+
+/// <summary>
+/// The default <see cref="IAssembler"/> backend. Hand-emits calling-convention
+/// trampolines and injection stubs using deterministic byte emitters
+/// (<see cref="EmitU8"/>, <see cref="EmitU32"/>, <see cref="EmitU64"/>). Has
+/// no native or third-party dependency — no FASM, no Iced.
+/// </summary>
+/// <remarks>
+/// <see cref="Assemble"/> is not supported by this backend (it is a parse-free
+/// emitter, not a text assembler). Use <see cref="IcedAssembler"/> (Phase 8) for
+/// arbitrary mnemonics.
+/// </remarks>
+public class StubAssembler : IAssembler
+{
+    /// <inheritdoc />
+    /// <exception cref="NotSupportedException">Always thrown. StubAssembler does
+    /// not parse text assembly; use IcedAssembler for that.</exception>
+    public byte[] Assemble(string assemblyText, ulong origin = 0)
+    {
+        throw new NotSupportedException(
+            "StubAssembler does not parse text assembly. " +
+            "Use IcedAssembler (Phase 8) for arbitrary mnemonics.");
+    }
+
+    // ── Emit primitives ────────────────────────────────────────────────────
+
+    /// <summary>Appends a single byte to <paramref name="buffer"/>.</summary>
+    public void EmitU8(List<byte> buffer, byte value)
+    {
+        buffer.Add(value);
+    }
+
+    /// <summary>Appends <paramref name="value"/> as 4 little-endian bytes.</summary>
+    public void EmitU32(List<byte> buffer, uint value)
+    {
+        buffer.Add((byte)value);
+        buffer.Add((byte)(value >> 8));
+        buffer.Add((byte)(value >> 16));
+        buffer.Add((byte)(value >> 24));
+    }
+
+    /// <summary>Appends <paramref name="value"/> as 8 little-endian bytes.</summary>
+    public void EmitU64(List<byte> buffer, ulong value)
+    {
+        EmitU32(buffer, (uint)value);
+        EmitU32(buffer, (uint)(value >> 32));
+    }
+}