Rewrite FieldsWillMerge to flatten fragments and deduplicate by signature

Replace the exponential recursive fragment cross-comparison algorithm
with a linear field-flattening approach based on:
https://tech.new-work.se/graphql-overlapping-fields-can-be-merged-fast-ea6e92e0a01

Instead of comparing fields-vs-fragments and fragments-vs-fragments
separately (O(n*m*k) where m=fragments, k=nesting depth), flatten all
fragment spreads into a single response-key map and compare within it.

Key optimizations in the new algorithm:
- Deduplicate fields by signature (name + definition + arguments) to
  avoid redundant comparisons — fields_merge benchmark: 3.1s to ~1.8ms
- selections_may_conflict? fast path skips validation when all direct
  children are unaliased unique-named fields with no fragments
- Cache collect_fields results per (node, return_type) to avoid
  re-expanding the same sub-selections across comparison contexts
- Track compared sub-selection node pairs to prevent infinite recursion
- Store single Field directly in response_keys hash, only wrap in
  array on collision (saves ~1200 array allocations)
- Lazy visited_fragments allocation (nil until first fragment spread)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: swalkinshaw