perf(validation): optimize field overlap check

[pavelnikolov]

Reduces worst‑case memory/time usage of overlapping field validation and adds a defensive configurable cap. Introduces benchmarks, fuzz coverage, and unit tests around the new behavior:

• Optimize overlapping field validation (group by response name, canonical pair cache, indexed child comparisons) to reduce quadratic blowups.
• Add configurable safety cap OverlapValidationLimit(n) schema option (rule: OverlapValidationLimitExceeded).
• Add benchmarks (large sibling fields + fragment spreads).
• Add fuzz test for mixed field/fragment patterns.
• Add unit test for limit behavior.

addresses #102