feat(onchain): add proptest invariant suite for crowdfund_vault

[Johnsmichael150]

feat(onchain): Add proptest invariant suite for crowdfund_vault

Summary

Introduces a property-based test suite (tests/invariants.rs) for the crowdfund_vault Soroban smart contract using the proptest crate. All changes are in the test layer — no production code was modified.

Motivation

Hand-written unit tests in src/test.rs cover specific examples but cannot exhaustively explore the input space. Property-based testing generates thousands of randomized scenarios to verify that core protocol invariants hold regardless of input, catching edge cases that example-based tests miss.

Changes

New file: apps/onchain/contracts/crowdfund_vault/tests/invariants.rs

Five property tests, each running 1000 cases:

Property	Test function	Validates
1 — Balance equals sum of contributions	prop_balance_equals_sum_of_contributions	Req 1.1, 1.5
4 — Contribution tracking round-trip	prop_contribution_tracking	Req 2.1
7 — Overdraft is rejected	prop_overdraft	Req 3.1
9 — Single-contributor match approximation	prop_single_contributor_match	Req 4.2
11 — Paused contract blocks all mutations	prop_paused_blocks_mutations	Req 5.1, 5.2, 5.3

Modified: apps/onchain/contracts/crowdfund_vault/Cargo.toml

Added proptest = "1" to [dev-dependencies].

Modified: apps/onchain/.gitignore

Excluded proptest-generated snapshot JSON files (**/test_snapshots/prop_*.json, **/test_snapshots/scaffold_*.json) to keep the repo clean.

How to run

# From apps/onchain/
cargo test -p crowdfund_vault --test invariants

Expected output: 6 tests pass (1 scaffold smoke test + 5 property tests).

Test coverage

• Property 1: Generates 1–5 distinct contributors with amounts 1–1,000,000. Asserts get_balance == Σ amounts and total_deposited == Σ amounts.
• Property 4: Generates a fresh contributor and deposit amount. Asserts get_contribution == prior + amount.
• Property 7: Generates deposit D and excess E > 0. Asserts try_withdraw(D + E) returns an error and balance remains D.
• Property 9: Generates a single contribution C (1–10,000,000). Asserts calculate_match is within 1% of C (fixed-point rounding tolerance for sqrt_scaled).
• Property 11: Pauses the contract. Asserts try_deposit, try_create_project, and try_withdraw all return ContractPaused and no state changes.

Notes

• Optional properties (2, 3, 5, 6, 8, 10, 12) were skipped per spec — they can be added incrementally.
• Each proptest! block constructs a fresh Env inside the closure because Soroban's Env is not Send.
• Contributor counts are kept to 1–5 per test run to stay within Soroban ledger limits.

Close #429

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[Cedarich]

Please fix failing workflow