feat(contract): add bounded yield accrual with explicit rate and timestamp semantics

[arisu6804]

Problem

Yield accrual can become ambiguous or overflow when timestamps, rate changes, or long inactive periods are not bounded.

Objective

Deliver a production-quality improvement to yield calculation and accrual state that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Define timestamp monotonicity, rate units, compounding or simple-accrual semantics, and maximum accrual interval.
• Prevent duplicate accrual for the same interval and handle clock anomalies safely.
• Expose the last-accrued timestamp and rate version in relevant events.

Acceptance criteria

• Accrual is deterministic for a given state and time.
• Backward timestamps and arithmetic overflow fail safely.
• Rate changes apply exactly at the documented boundary.

Required validation

• Boundary and property tests for zero/maximum rates, long intervals, rate changes, and repeated calls.
• 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 on this issue and explain design tradeoffs.
• Include test evidence, compatibility impact, and migration or rollout notes where relevant.
• 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.