Set up automated contract fuzz testing pipeline

[Smartdevs17]

Description

Manual testing misses edge cases. Set up automated fuzz testing for smart contracts in CI.

Acceptance Criteria

• cargo-fuzz targets for subscription functions
• CI fuzz run with minimum duration (30min)
• Corpus management with seed generation
• Crash triage and regression test generation
• Coverage-guided fuzzing
• Documentation for adding new fuzz targets

Technical Scope

• contracts/tests/fuzz.rs
• .github/workflows/fuzz-test.yml
• contracts/subscription/FUZZING.md
• Edge: fuzzing timeout handling, CI resource allocation, non-deterministic crashes