feat(contract): define atomic and partial semantics for batch operations

[arisu6804]

Problem

Batch callers cannot safely retry when partial application and item results are unspecified.

Objective

Deliver a production-quality improvement to batch transfer, escrow, and release entrypoints that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Document atomic or itemized behavior; bound batch size; return stable indexed results and enforce idempotency.

Acceptance criteria

• Mixed-validity batches are deterministic; no item is applied twice; result ordering and error codes are stable.

Required validation

• Mixed-batch, duplicate, limit, rollback, ordering, 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.