feat(contract): version stream lifecycle event schemas

[arisu6804]

Problem

Indexers cannot reconstruct stream balances if event fields and timestamp or amount units change implicitly.

Objective

Deliver a production-quality improvement to create, top-up, cancel, withdraw, and pause events that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Define versioned event schemas with stream ID, actor, participants, amounts, status, timestamps, and outcome; document units.

Acceptance criteria

• Every lifecycle event is parseable and unambiguous; incompatible schema changes are detected by fixtures.

Required validation

• Event fixture, parser compatibility, and end-to-end emission 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.