refactor(where): consolidate reflection cache into partial CachedMethods

np.where's IL kernel had ~35 MethodInfo fields scattered across the file using
`Array.Find(...)!` null-forgiveness, which throws NullReferenceException at first
use if a framework method ever gets renamed/removed. The existing CachedMethods
nested class in ILKernelGenerator.cs follows a fail-fast `?? throw new
MissingMethodException(type, name)` pattern, keyed per MethodInfo, and is the
project convention for every other kernel partial.

Changes:

- Make `CachedMethods` a `partial` nested class so Where-specific reflection can
  live alongside the kernel file it serves. (ILKernelGenerator.cs: 1 line.)

- Delete the 35 `_v128*/_v256*/_avx2*/_sse41*` private fields from
  ILKernelGenerator.Where.cs and move them into a new "Where Kernel Methods"
  region inside a partial `CachedMethods` declaration at the bottom of that
  file. Renamed to PascalCase (e.g. _v256LoadByte -> V256LoadByte) to match the
  existing CachedMethods naming convention.

- Introduce three small helpers inside CachedMethods:
  - FindGenericMethod(Type, string name, int? paramCount) - wraps the
    `Array.Find(GetMethods(), m => m.IsGenericMethodDefinition && ...)` pattern
    with a MissingMethodException fail-fast throw. Handles the overload count
    disambiguation for Load/Store.
  - FindMethodExact(Type, string name, Type[] argTypes) - wraps GetMethod with a
    fail-fast throw. Used for Avx2/Sse41 specific overloads.
  - GetZeroGetter(Type vectorOfT) - wraps Property("Zero").GetMethod with a
    fail-fast throw. Used for the 8 Vector*<T>.Zero getters.

- Update all 41 call sites in EmitInlineMaskCreationV256/V128 and
  EmitWhereV256/V128BodyWithOffset to use CachedMethods.Xxx.

Behaviour unchanged; 180 np.where + np.asanyarray tests still pass on net8.0 +
net10.0. The single real benefit is earlier and clearer failure if any of the
~35 framework API names change in a future .NET release.

Author: Nucs