testing: add fuzz/property tests for parsers and consensus invariants

## Goal
Add fuzzing and property tests to catch edge cases and ensure invariants.

## Scope
- Fuzz message parsing / decoding
- Property tests for determinism and convergence conditions
- Regression corpus for past failures

## Acceptance criteria
- Fuzz targets run in CI (time-bounded)
- Property tests cover key invariants (no negative balances, monotonic nonces, etc.)