[Ranjeet2063]

Summary of Changes

Implements a comprehensive SMT-based formal verification engine using the Z3 theorem prover for Stellar Soroban smart contracts:

• Phase 1: SMT-LIB2 Bitvector Encoding: Translates contract arithmetic and control flow into bitvector logic ((_ BitVec 32), (_ BitVec 64), (_ BitVec 128) and QF_ABV).
• Phase 2: Invariant Specification Language: Doc-comment parser (/// #[invariant: ...]) for state transitions and temporal assertions.
• Phase 3: Soroban Host Environment Theory: Formalizes env.storage(), require_auth(), and ledger timestamps as SMT theories and uninterpreted functions.
• Phase 4: Modular Verification: Function summaries (FunctionSummary) decoupling multi-contract interactions.
• Phase 5: Loop Handling: Bounded unrolling (up to max_unroll = 20) and inductive invariant verification.
• Phase 6: Counterexample Generation: Maps SAT solver models back to concrete Soroban input parameters and reproduction traces.
• Phase 7: Structured JSON Reporting: Machine-readable verification outputs.
• Phase 8: 15-Contract Benchmark Suite: 5 provably correct invariants (UNSAT), 5 invariant violations with counterexamples (SAT), and bounded loop step verification.
• Documentation: Comprehensive guide at docs/FORMAL_VERIFICATION.md.

Verification Proof

• Passed 5/5 formal verification unit and benchmark tests (100% Green CI).
• Verified via cargo test --lib formal_verification.
• Clean compilation confirmed via cargo check --tests.

Closes #445

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