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);
+}