[Quality][Medium] Make fixed-duration bond early-exit penalties deterministic

[Baskarayelu]

Objective

Implement one deterministic penalty schedule with checked arithmetic and explicit boundary behavior.

Why this matters

Early exit combines elapsed time, penalty tiers, and payout rounding. Ambiguity can produce inconsistent user outcomes or value leakage.

This is a substantive production-quality improvement. It must change runtime behavior, security guarantees, correctness, reliability, or meaningful user functionality. It is not a documentation-only, formatting-only, or trivial dependency task.

Scope

Area: fixed-duration bonds

Starting points: docs/fixed-duration-bond.md, docs/early-exit.md, fixed-duration bond crate

The contributor should verify the current implementation before changing it and keep the PR limited to this issue. Do not introduce unrelated refactors or weaken existing CI/security gates.

Acceptance criteria

• Penalty tier selection is unambiguous at every duration boundary.
• Penalty never exceeds principal or produces a negative payout.
• The same inputs produce the same payout regardless of call ordering.
• Closed bonds cannot be exited again.

Required validation

• Test each tier boundary, zero elapsed time, maturity, and maximum duration.

• Add randomized conservation tests for payout plus penalty.

• Test repeated and unauthorized early exits.

• The PR explains the failure mode, the chosen design, backward-compatibility impact, and rollback or migration considerations.

• The PR includes CI evidence and does not contain secrets, generated noise, unrelated cleanup, or disabled checks.

Contributor deliverables

• Open a focused feature branch and do not begin implementation until assigned.
• Reference this issue with Closes #<issue-number> or Fixes #<issue-number>.
• Check off every acceptance criterion in the PR with links to the relevant code and tests.
• Include a security/correctness note explaining why adversarial inputs cannot bypass the new guarantee.

Maintainer quality bar

The PR must be independently reviewable, preserve existing behavior outside this scope, exercise failure paths, and pass the repository's complete required CI/CD checks. Reward eligibility is not guaranteed by this issue or by merging.

[guptakumarranjeet150]

Hello! I'd like to work on this issue.

I can tackle [Quality][Medium] Make fixed-duration bond early-exit penalties deterministic. Proposed implementation roadmap:

• Analysis: Inspect current behavior and edge cases in CredenceOrg/Credence-Contracts
• Implementation: Implement [Quality][Medium] Make fixed-duration bond early-exit penalties deterministic with explicit require_auth checks, clean storage layouts, and robust Soroban contract logic.
• Testing: Add comprehensive unit tests verifying both happy-path invocations and unauthorized revert scenarios.
• PR: Atomic commit with clean git hygiene and passing CI

Happy to pick this up today—please assign to me!

[shobhamerabacha-star]

Hi everyone,

I would love to take this on. My planned approach:

• Implement [Quality][Medium] Make fixed-duration bond early-exit penalties deterministic with explicit require_auth checks, clean storage layouts, and robust Soroban contract logic.
• Add comprehensive unit tests verifying both happy-path invocations and unauthorized revert scenarios.

I am ready to start immediately—please assign this to me!