feat: cr8-usd <-> musd parity swap router for cross-vertical settlement (#83)

[Ishant5436]

Fixes #83. Implements StableSwapRouter.sol for 1:1 parity conversion with custom epoch rate limits and protocol fees. Includes switchboard/swap.py client and full test suites for both Solidity and Python.

[abhicris]

Welcome to kcolbchain, @Ishant5436 — glad you're here. 🌱

Here's what happens from this PR:

1. Our automated review looks for obvious issues (tests, secrets, size) within a couple of hours.
2. If it's clean and CI passes, we merge without back-and-forth.
3. If we need changes, we'll leave a specific comment — not a generic nit. Push another commit and we re-review.

While you wait:

• Run the repo's tests locally (see the repo README.md).
• Keep the PR scoped to one concern — bigger PRs land slower.
• Don't commit tokens or .env contents.

What happens after your first merge

• You're listed on https://kcolbchain.com/contributors/ (opt-out anytime via data/contributors-optout.txt).
• At 2+ merged PRs you're a Regular; at 5+ you're a Fellow — Fellows are the pool we invite first for paid research, partner-org gigs, and grant-funded work. Details: https://kcolbchain.com/docs/org-contribution-policy.html
• Looking for what to pick up next? https://kcolbchain.com/invitations/ lists open help-wanted / good-first-issue across every kcolbchain repo.

Thanks for writing the code. We're building this to last.

[Pattermesh]

Hold — requesting security review before merge.

This PR introduces StableSwapRouter.sol with CR8-USD <-> MUSD parity swap logic and epoch rate limits. Given this is a financial contract handling cross-vertical settlement, we'd like a human security review before merging. The automated pipeline safe verdict is insufficient for production-bound financial contract code.

@abhicris — could you review the Solidity in this PR? Key areas:

• Parity enforcement mechanism
• Epoch rate limit bypass vectors
• Access control on the router

[abhicris]

🤖 Audit verdict: safe

Well-structured parity swap router using battle-tested OpenZeppelin patterns; no malicious code, proper access controls (onlyOwner), reentrancy guards, SafeERC20 usage, input validation, and comprehensive tests.

Audited by the kcolbchain PR pipeline. See pipeline docs.