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).