feat: implement Soroban router contract for atomic vault rebalancing

[collinsezedike]

Summary

• Added packages/contracts/router/ with a MeridianRouter Soroban contract exposing a single rebalance(depositor, from_vault, to_vault, shares, min_out, route_to) entry point that withdraws from one Meridian vault and deposits into another within a single transaction.
• depositor.require_auth() is called before any sub-invocations, so Soroban's auth tree enforces the depositor's signature over both legs atomically.
• A slippage guard panics (reverting the whole transaction) when the withdrawal returns fewer stroops than min_out, ensuring the depositor cannot receive less than they specified.
• A minimal VaultInterface trait drives the cross-contract calls via contractclient, so no vault WASM is compiled into the router.
• Added two Rust unit tests using soroban-sdk::testutils: one verifies funds move correctly between vaults, the other confirms the slippage guard reverts as expected.
• Updated scripts/deploy-testnet.sh to build and deploy the router alongside the vault contract.
• Added docs/contracts.md documenting the contract interface, auth model, and slippage behaviour.
• Updated packages/contracts/Cargo.toml to include the router as a workspace member.

Test plan

• pnpm lint && pnpm typecheck && pnpm test pass locally
• cargo test -p meridian-router passes with both test cases green
• Slippage test panics with "slippage" message when min_out exceeds withdrawal amount
• scripts/deploy-testnet.sh deploys the router and prints its contract ID without error

Closes #15

[vercel]

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Project	Deployment	Actions	Updated (UTC)
meridian	Ready	Preview, Comment	Jul 5, 2026 6:39pm