feat(offer-liability-headroom): audit offer liabilities against account balances

[Caneryy]

Summary

Adds a read-only Offer Liability and Capacity Audit that pairs Horizon offers with account balances/liabilities.

• Associates each offer with buying and selling asset identities
• Shows balance, selling/buying liabilities and trustline limit separately
• Keeps exact price_r rationals (no float math for capacity estimates)
• Warns on unmatched assets and ledger skew; large skew fails as inconsistent
• Never implies estimates guarantee execution

Files

Entire slice under features/offer-liability-headroom/ (35 files).

Out of scope

• No offer creation/cancellation
• No market prediction
• No changes outside this feature folder

Test plan

• npm run verify:features -- offer-liability-headroom
• npm test -- --run features/offer-liability-headroom

Closes #471

[drips-wave]

@Caneryy Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits