feat(contract): define batch operation rollback and result semantics

[arisu6804]

Problem

Batch callers cannot safely retry when partial application and per-item results are undefined.

Objective

Deliver a production-quality improvement to batch create, top-up, and cancellation operations that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Choose atomic or itemized semantics; return stable indexed outcomes; enforce idempotency and bounded batch size.

Acceptance criteria

• Mixed batches produce deterministic documented results; retries do not duplicate funds or streams; batch work stays within limits.

Required validation

• Mixed-validity, duplicate, rollback, ordering, limit, and retry tests.
• Existing tests and CI remain passing.
• Add regression coverage for the original failure mode.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Keep the PR focused and explain design tradeoffs, compatibility impact, and test evidence.
• Avoid typo-only, documentation-only, cosmetic-only, or unrelated refactor submissions.

Out of scope

• Broad rewrites not required by the acceptance criteria.
• Changes to unrelated services, contracts, or user flows.