test(governance): add proptest for quorum and timelock invariants (#635)

[Killerjunior]

Pull Request

Description

Adds a property-based test suite for the FluxoraGovernance contract's approve/execute/QuorumReachedAt logic using the proptest crate. The suite randomises signer-set sizes, approval orderings, and time advances across 8 invariant properties to assert that:

1. A proposal never executes below quorum — execute always returns QuorumNotReached when approvals < threshold, regardless of elapsed time.
2. A proposal never executes before the timelock elapses — execute always returns TimelockNotElapsed when now < quorum_at + GOVERNANCE_TIMELOCK_SECONDS.
3. The executed flag is one-way — a second execute on the same proposal always returns AlreadyExecuted.

Boundary conditions at approvals == threshold and now == quorum_at + TIMELOCK are explicitly probed to catch off-by-one bugs. The full (quorum × timelock) truth-table is also covered via a cross-product property.

Documentation in docs/governance.md is extended with a dedicated Property-Based Tests section describing each invariant, security notes, and how to run the suite.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test coverage improvement
• Refactoring (no functional changes)

Related Issues

Closes #635

Changes Made

• Added contracts/stream/tests/governance_proptest.rs — new proptest-driven integration test module with 8 invariant properties across 5 proptest! blocks (256 cases each, deterministic via fixed source_file seed)
• Extended docs/governance.md with a Property-Based Tests section documenting all invariants, security notes on the timelock and quorum boundaries, and instructions to run/reproduce failures

Testing

Test Coverage

• All tests pass locally: cargo test -p fluxora_stream
• New tests added for new functionality
• Existing tests updated for changed functionality
• Test coverage remains above 95%

Test properties added

Property	Invariant
prop_execute_fails_below_quorum	QuorumNotReached whenever approvals < threshold
prop_execute_fails_before_timelock_elapses	TimelockNotElapsed for now < quorum_at + TIMELOCK
prop_execute_succeeds_at_timelock_boundary	Success at exactly now == quorum_at + TIMELOCK
prop_execute_succeeds_after_timelock_boundary	Success for all now > exec_after
prop_second_execute_always_returns_already_executed	AlreadyExecuted on any repeat call
prop_execute_outcome_matches_quorum_and_timelock_conditions	Full (quorum × timelock) truth table
prop_exactly_quorum_approvals_allows_execution	Exactly threshold approvals + boundary time succeeds
prop_one_below_quorum_prevents_execution	threshold - 1 approvals always blocks regardless of time

Run the suite with:

cargo test --test governance_proptest --package fluxora_stream

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[Jagadeeshftw]

the quorum and timelock invariant proptests are exactly the kind of coverage governance needs, and the formal-verification smoke test plus the adversarial-auth test round it out well. rebased onto the latest main and it merged cleanly.