Add quote_fee_for_anchor(anchor, asset, amount) so fee-waived anchors get an accurate zero-fee preview

[Jagadeeshftw]

📌 Description

quote_fee(env, asset, amount) in src/lib.rs computes amount * effective_fee_bps(asset) / BPS_DENOMINATOR with no anchor parameter, so it always reports a nonzero fee for an asset with a nonzero rate — even for an anchor that currently holds an active set_fee_waiver, which open_settlement charges 0 fee via its own storage::is_fee_waived check. Any integrator previewing cost for a waived anchor via quote_fee today gets an overstated number.

🧩 Requirements and context

• Add quote_fee_for_anchor(env, anchor, asset, amount) -> Result<i128, Error> that checks storage::is_fee_waived first, mirroring open_settlement's fee branch, and falls back to the existing effective_fee_bps calculation otherwise.
• Keep quote_fee's existing (anchor-agnostic) signature and behavior unchanged for backward compatibility.
• Add a regression test asserting quote_fee_for_anchor returns 0 for a waived anchor and matches quote_fee for a non-waived one.

🛠️ Suggested execution

• Add the new entrypoint to src/lib.rs, sharing logic with quote_fee and open_settlement's fee computation where practical.
• Add src/test.rs coverage for a waived anchor, a non-waived anchor, and an asset with a set_asset_fee override combined with a waiver.
• Document in the README that quote_fee is anchor-agnostic while quote_fee_for_anchor accounts for waivers.

✅ Acceptance criteria

• quote_fee_for_anchor returns 0 whenever the anchor holds an active fee waiver, regardless of the asset's configured rate.
• For a non-waived anchor, quote_fee_for_anchor matches quote_fee's existing result.
• The fee actually accrued by a subsequent open_settlement/execute_settlement matches the anchor-aware preview.

🔒 Security notes

A quote that overstates cost is a minor trust issue, but a quote that understates cost (the inverse bug) would let an integrator commit to a settlement based on a wrong number; locking in an accurate anchor-aware preview closes that risk in either direction.

📋 Guidelines

• Minimum 95% test coverage
• Clear documentation
• Timeframe: 96 hours

[Delightech28]

I'd like to work on

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #117)
2. Your PR will be reviewed by the AnchorNet-Org maintainers

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