[Docs] contracts/fuzz_invariants.md is not linked from contracts/README.md

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Why this matters
contracts/fuzz_invariants.md documents the invariants each fuzz target checks, but contracts/README.md never references it, so contributors setting up fuzzing via contracts/FUZZING_README.md and contracts/fuzz_campaign.sh have no obvious path to find it.

Acceptance criteria

• Add a link to fuzz_invariants.md from contracts/README.md
• Cross-link contracts/FUZZING_README.md and fuzz_invariants.md to each other

Files to touch

• contracts/README.md
• contracts/FUZZING_README.md

Out of scope
Adding new invariants.