[Refactor] NDIterator to IL generation with Expression/IL/Delegate kernel injection

[Nucs]

Problem

NumSharp has 14+ iterator/incrementor types with significant issues:

• 3 types are dead code (0 usages)
• ~23 files, ~4,600 lines of duplicated iteration logic
• Delegate overhead: 4-5 closures per NDIterator
• No kernel fusion: Operations applied outside iteration loop
• No SIMD integration: Sequential element-by-element only

---

Solution: Unified NDIterator with 3-Tier Kernel System

Tier 1: Static Interfaces (Zero overhead, SIMD)

public interface IUnaryKernel<TIn, TOut>
{
    static abstract TOut Apply(TIn value);
    static virtual int ApplyVector(ReadOnlySpan<TIn> input, Span<TOut> output) => 0;
}

public interface IReductionKernel<TIn, TAccum>
{
    static abstract TAccum Identity { get; }
    static abstract TAccum Combine(TAccum accumulator, TIn value);
    static virtual bool ShouldContinue(TAccum accumulator) => true; // Early-exit
}

// Usage
NDIterator.Transform<double, double, SquareKernel>(source, dest);
NDIterator.Reduce<double, double, SumKernel>(source);

Tier 2: IL Injection (Full control)

// Sigmoid: 1 / (1 + exp(-x))
NDIterator.Transform<double, double>(arr, result, (il, ctx) =>
{
    il.Emit(OpCodes.Neg);
    il.EmitCall(OpCodes.Call, typeof(Math).GetMethod("Exp"), null);
    il.Emit(OpCodes.Ldc_R8, 1.0);
    il.Emit(OpCodes.Add);
    il.Emit(OpCodes.Ldc_R8, 1.0);
    il.Emit(OpCodes.Div);
});

Tier 3: Func Delegates (Simple, ~10-15 cycle overhead)

NDIterator.Transform(source, dest, x => x * x);
NDIterator.Reduce(source, (acc, x) => acc + x, identity: 0.0);
NDIterator.Mask(source, x => x > 0.5);

---

Kernel Interfaces (7 total)

Interface	Purpose	SIMD
IUnaryKernel<TIn, TOut>	Element transform	ApplyVector
IBinaryKernel<TL, TR, TOut>	Binary element-wise	ApplyVector
IReductionKernel<TIn, TAccum>	Sum, Prod, Min, Max + early-exit	CombineVector
IIndexedReductionKernel<TIn, TAccum>	ArgMax, ArgMin	-
IAxisKernel<TIn, TOut>	CumSum, CumProd (stride support)	Manual
ITernaryKernel<T>	np.where	ApplyVector
IPredicateKernel<T>	Boolean masking	ApplyVector

---

Performance

Tier	Overhead	JIT Inline	SIMD	Use Case
1 (Interface)	~0	Yes	Via ApplyVector	Hot paths
2 (IL Inject)	~0	Full control	Manual	Custom ops
3 (Func<>)	~10-15 cycles	No	No	Prototyping
Old NDIterator	~10-15 cycles	No	No	Eliminated

---

Migration

Old:

var iter = new NDIterator<double>(source, false);
while (iter.HasNext()) sum += iter.MoveNext() * iter.MoveNext();

New:

sum = NDIterator.Reduce<double, double, SumOfSquaresKernel>(source);
// Or simple:
sum = NDIterator.Reduce(source, (acc, x) => acc + x * x, 0.0);

---

Scope

• 23 files deleted (iterators + incrementors)
• 7 new files (~3,000 lines)
• Out of scope: Type promotion (caller's job), broadcasting (caller provides), multi-output ops

---

Full Design

See docs/UNIFIED_ITERATOR_DESIGN.md for:

• Complete interface definitions with examples
• 10+ IKernelEmitter examples (Power, Clip, Sigmoid, ReLU, etc.)
• IteratorState struct specification
• Stack contracts for IL emitters
• Built-in kernel implementations