Fix three code bugs

[psyk0cyber]

Fixes three bugs: replaces transfer with sendValue for native ETH, hardens isFraction against null/primitives, and switches assert to require in TestBNT.sol.

The changes improve robustness by preventing gas stipend issues in native token transfers, avoiding runtime crashes in a type guard, and providing clearer error handling for external call failures in a test contract.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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