Property-based tests for split-stream weight allocation

[gbengaeben]

Category: Testing | Priority: Low | Dependencies: none | Out of scope: new split entrypoints

Description & Context: Split dust math (largest-remainder) has unit tests but no randomized proptest — the settlement invariant only fuzzes single streams. Weight/dust edge cases deserve the same treatment.

Acceptance Criteria / DoD:

• proptest over random (total_amount, weights) for both _bps and _pct: shares sum to exactly total; every share > 0; no share > total; deterministic across runs (same seed)
• Property test: per-member created events' amounts sum to the single pulled total
• Run in CI (rust.yml) alongside the existing proptest suite

How to verify: cargo test -p stream-core split — new properties run thousands of random cases, all green.