Clarify mock transform algorithms

[magicmark]

Summary

• clarify TransformOperation so it starts from an operation definition instead of treating SelectionSet nodes as directive-bearing
• split mocked-selection removal into TransformSelectionSet and keep follow-up pruning as a fixed-point cleanup
• clarify unused variable removal after mocked selections and fragment pruning
• fix ValidateNoNestedMocks to propagate mocked-parent state with OR semantics
• keep fragment expansion readable in pseudocode while noting recursion guards

Notes

This matches the interpretation used by the Apollo Client demo implementation: operation-level @mock bypasses the server, field-level @mock removes the field selection, empty parents/fragments are pruned afterward, and children of a mocked field cannot also use @mock across fragment boundaries.

Validation

• git diff --check

Draft PR: This PR was sent by Claude and may not have yet been reviewed by markl.