Add formal invariant assertion suite enforcing sum(investments) <= sum(invoice amounts)

[Baskarayelu]

Description

A protocol-wide solvency invariant is that the total of all active investments held in escrow must never exceed the sum of accepted invoice amounts denominated per currency. This issue introduces a stateful test that asserts this invariant after every public state transition driven by a randomized scenario, and exposes a runtime validate_solvency_invariant(env) admin helper that can be wired into a periodic off-chain check.

Requirements and context

• Secure: helper is admin-gated and read-only.
• Tested: invariant checked after every transition in the model.
• Documented: docs/solvency-invariant.md.
• Reference: src/lib.rs, src/investment.rs, src/payments.rs.

Suggested execution

git checkout -b feature/solvency-invariant

• Add src/invariants.rs::validate_solvency_invariant.
• Add src/test_solvency_invariant.rs driving randomized lifecycles.
• Docs: docs/solvency-invariant.md.
• Rust doc comments stating the invariant in plain language.
• Validate that pause, default, and dispute paths preserve the invariant.

Test and commit

Run cargo test test_solvency_invariant -- --nocapture. Cover edge cases: default refunds, dispute resolutions, partial settlements. Include security note classifying any violation as a P0.

Example commit message

feat(invariants): add protocol-wide solvency invariant and stateful enforcement tests

Guidelines

• Minimum 95% test coverage
• Clear documentation in docs/solvency-invariant.md
• Timeframe: 96 hours