Add create-tap-with-borrowed-liquidity convenience function

[warmidris]

Summary

• Adds create-tap-with-borrowed-liquidity to contracts/reservoir.clar, combining create-tap and borrow-liquidity into a single transaction
• Adds two tests: happy path (verifies balances and expiry block) and error path (invalid signature)

Motivation

In the common setup flow for a new participant (e.g. a stackmail agent opening a mailbox), they need to:

1. Open a tap by depositing funds for outgoing capacity
2. Borrow liquidity from the reservoir for incoming capacity

Currently this requires two separate transactions. This convenience function combines them atomically, reducing the on-chain footprint and simplifying client-side setup code.

Parameters

Same as create-tap + borrow-liquidity combined:

• stackflow / token / tap-amount / tap-nonce → forwarded to create-tap
• borrow-amount / borrow-fee / my-balance / reservoir-balance / my-signature / reservoir-signature / borrow-nonce → forwarded to borrow-liquidity

Returns

(ok expire-block) — the burn block height when the borrowed liquidity expires (same as borrow-liquidity).

Test plan

• Happy path: creates tap + borrows, verifies STX balances and return value
• Invalid signature: passes wrong signing key, expects (err u125) (InvalidSignature)
• Tests can't run locally (Node v18, clarinet-sdk requires v20) — CI should cover this

🤖 Generated with Claude Code

[obycode]

lgtm