Simplify Fabric Differentiator mutation calculation (#57153)

Summary:

`calculateShadowViewMutationsFlattener` (CCN 73) and `calculateShadowViewMutations` (CCN 45) are the core of the Fabric tree differ and were the two most complex functions in the mounting library. This is a pure, behavior-preserving refactor: cohesive blocks are extracted into file-local `static` helpers (remove/insert-on-(un)flatten, matched-grandchildren update, opposite/normal reparent handling, and the greedy diff stages), turning both functions into flat sequences of helper calls. Traversal order and the exact sequence of emitted mutations are preserved. Only the `.cpp` is touched; no header or public API change.

Changelog: [Internal]

Differential Revision: D108027813

Author: sammy-SC