Property-based invariants: stream balance and escrow never negative

[greatest0fallt1me]

Property tests for invariants: non-negative balance and conservation of value

    ## Description

    Add **property** tests (e.g. fast-check, gopter, or QuickCheck) that randomize a

sequence of valid mutating events on a stream and assert invariants:
balances never negative; sum of legs matches on-chain mock; settle cannot
over-release escrow. Complements unit tests for a Soroban-backed system.

    ## Requirements and context

    - **Model** a simplified in-memory Stellar/escrow mock for speed.

• Seed-deterministic randomness for CI stability.

• Invariants listed in a single module as pure functions; reused in prod code path where possible.

• Failing cases shrink to minimal repro; commit as regression test.

• Time budget in CI; skip heavy mode on PR, run nightly.

    ## Suggested execution
  
    1. `git checkout -b test/property-invariants-balance`
  

2. Wire fast-check in JS or equivalent in your stack.

3. PR with 1–2 minimal shrunk failures turned into static tests.

4. Security note: invariants are not a substitute for on-chain security.

5. Link to any formal verification future work in contracts repo.

    ## Test and commit
   

• Run the full test suite; add or update tests until the agreed coverage bar is met.
• Cover edge cases listed in this issue; document any intentional exclusions with brief rationale in the PR.
• Include relevant test output (e.g. test runner summary) or a link to a passing CI run in the pull request.
• Add security notes for auth, keys, PII, chain settlement, or money movement (assumptions verified, out-of-scope items).

Example commit message

test(streams): property-based invariants for balances and escrow conservation

Guidelines

• Target: at least 95% coverage on new or meaningfully changed code (per the repo’s standard tooling).
• Documentation: update contributor-facing or API documentation where a reviewer would be blocked without it.
• Timeframe: 96 hours to ready-for-review (surface blockers early).