Chaos harness expansion to vault, splitter, batch-sender, governance

[truthixify]

Tier: M (2-4 days) | Type: test

Context. Wave 7 built the chaos harness (stellar/integration-tests/tests/harness.rs + chaos.rs, 950 lines total) but only wraps the four "core" contracts (announcer, sender, registry, names). Snapshot files under stellar/integration-tests/test_snapshots/ confirm the coverage stops there. The secondary contracts (stealth-vault, stealth-splitter, stealth-batch-sender, and the governance PoC) still hit a happy-path Soroban RPC in tests, so retry / bail policies for their entrypoints are undocumented and unverified.

Scope.

• Add ChaosClient wrappers for vault (deposit / claim / refund), splitter (create_split / fund_split), batch-sender (batch_send), and governance (propose / vote / execute).
• Add lifecycle tests through the harness for each contract, matching the shape of the existing announcer / sender / registry / names entries.
• Document per-entrypoint retry / bail policy in doc comments on each wrapper (matching the four already-wrapped contracts).
• Add the new tests to the nightly stellar-chaos-nightly CI job with WRAITHCHAOS_MODE=1 WRAITHCHAOS_FAILURE_RATE=0.3.

Acceptance.

• Four new snapshot files land under stellar/integration-tests/test_snapshots/ covering vault / splitter / batch-sender / governance "through_chaos"
• Every wrapped entrypoint has a documented retry / bail policy in its doc comment
• stellar-chaos-nightly CI job passes on the new tests
• stellar/integration-tests/FLAKE_HANDLING.md extended with the four new contracts

Files. stellar/integration-tests/tests/harness.rs, stellar/integration-tests/tests/chaos.rs, stellar/integration-tests/test_snapshots/ (new files), stellar/integration-tests/FLAKE_HANDLING.md, .github/workflows/ci.yml.