Add has_asset_fee_override FIX

[Mitch5000]

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PULL REQUEST — PLAIN TEXT FORMAT

Title: Add has_asset_fee_override to resolve fee-override ambiguity
Repo: Mitch5000/AnchorNet-Contracts
Branch: fix/asset-fee-override-visibility -> main
Status: READY FOR REVIEW

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FILES MODIFIED (3) — NO NEW FILES CREATED

1. src/lib.rs
2. src/test.rs
3. README.md

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ISSUE ADDRESSED

asset_fee(env, asset) collapses storage::get_asset_fee Option into a plain
u32 via effective_fee_bps. A caller cannot distinguish:

• explicit override to 0 bps (set_asset_fee(asset, 0))
• no override with global fee() == 0

This is the same "collapsed Option" ambiguity that motivated is_anchor_active
for anchors, applied to fee overrides.

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FIX IMPLEMENTED (ADDITIVE, NON-BREAKING)

• Added entrypoint: has_asset_fee_override(env, Env, asset, Symbol) -> bool
  Delegates directly to: storage::get_asset_fee(&env, &asset).is_some()
• Did NOT modify asset_fee existing behavior.
• Added regression tests in src/test.rs (4 cases):
  • false by default (no override + global fee 0)
  • true after set_asset_fee(asset, 0) (explicit zero override)
  • true for nonzero override
  • false after clear_asset_fee
• Documented distinction in README.md fee section.

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ACCEPTANCE CRITERIA

[YES] has_asset_fee_override returns true only when set_asset_fee called
and not yet cleared for that asset.
[YES] clear_asset_fee flips result back to false.
[YES] asset_fee existing return value and callers unaffected.
[YES] Confidence: 100% (additive, non-breaking, mirrors existing patterns).

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Closes #107