Invariant test: Σ locked_tokens + Σ claimed_tokens + Σ surplus == total_funded #223

Problem Statement. The Soroban contract tracks KEY_TOTAL_LOCKED and KEY_TOTAL_CLAIMED
per token but never asserts the invariant that those totals plus the surplus equal the
total ever funded. A single off-by-one in decrement_locked() or finalize_claim() would
corrupt the ledger silently — and there is no on-chain total_funded storage key, so the
test must reconstruct that aggregate from the EscrowFunded event stream.

Why it matters. Mis-bookkeeping on a humanitarian escrow is what auditors will look at
first. Even a sub-stroop drift compounds through the lifetime of the contract.

Technical Context. app/onchain/contracts/aid_escrow/src/lib.rs keeps two Map<Address, i128> totals. get_total_locked and get_total_claimed are exposed to readers but never
asserted against one another, and the contract emits EscrowFunded { from, token, amount, timestamp } events from fund() and SurplusWithdrawnEvent / PackageRefunded events
elsewhere. Invariant #15 reconstructs total_funded by summing those events in a Soroban
test harness.

Expected Outcome. A proptest!-driven invariant test that exercises
fund → create → claim → fund → create → revoke / refund / withdraw_surplus triples and
asserts

Σ locked(tokens) + Σ claimed(tokens) == Σ EscrowFunded.amount(tokens)
                                    − Σ SurplusWithdrawn.amount(tokens)
                                    − Σ transfers-to-admin (refunded)

Acceptance Criteria.

• New file app/onchain/contracts/aid_escrow/tests/invariants.rs aggregates events using
  env.events().all() and replays them post-hoc, then runs 100 random sequences.
• Failure produces a minimal shrunk example identifying which instruction broke it,
  along with a printed event-stream snapshot.
• The invariant is also asserted at bookkeeping calls (decrement_locked,
  finalize_claim, withdraw_surplus, refund) using unit-level smoke tests.

Implementation Notes. Use proptest as a dev-dependency. Seed proptest! with a
fixed seed in CI for reproducibility. Events are emitted via the contractevent macro —
use env.events().all() (or the equivalent in the chosen SDK) at assertion time.

Files or modules likely to be affected. app/onchain/contracts/aid_escrow/tests/invariants.rs.

Dependencies. None.

Difficulty. Medium
Estimated effort. M

Backlog item #15 from `docs/maintainer-issue-backlog.md.