refactor: replace hardcoded dust limits with BDK's is_dust()

[darioAnongba]

Summary

Replace the hardcoded getDustLimitSats switch statement in validation.ts with BDK's Amount.is_dust(script) method, which computes dust thresholds from the actual script — matching Bitcoin Core's relay policy exactly.

Before

function getDustLimitSats(addressType: AddressType): bigint {
  switch (addressType) {
    case 'p2wpkh': return 294n;
    case 'p2pkh':  return 546n;
    case 'p2sh':   return 540n;
    case 'p2wsh':  return 330n;
    case 'p2tr':   return 330n;
    default:       return 546n;
  }
}

After

const amount = Amount.from_btc(Number(amountInBtc));
if (amount.is_dust(account.publicAddress.script_pubkey)) { ... }

Benefits

• Correct by construction — delegates to BDK / bitcoin-core's dust relay logic
• Future-proof — automatically handles new script types without code changes
• Less maintenance — no hardcoded constants to keep in sync
• Simpler — removes ~25 lines, replaces with 2

Dependency

This requires @metamask/bitcoindevkit to be built from bitcoindevkit/bdk-wasm v0.3.0+ (release PR: bitcoindevkit/bdk-wasm#41), which exposes:

• Amount.is_dust(script) — check if amount is below dust for a script
• ScriptBuf.minimal_non_dust() — get the minimum non-dust amount for a script
• ScriptBuf.minimal_non_dust_custom(fee_rate) — same with custom dust relay fee

These were added in bitcoindevkit/bdk-wasm#13.

Testing

Existing dust limit tests in RpcHandler.test.ts (lines 605-642) should continue to pass since the BDK computation produces the same thresholds (294 sats for p2wpkh) as the previously hardcoded values.

---

Note

Medium Risk
Changes send-amount validation behavior by delegating dust checks to BDK’s Amount.is_dust(script_pubkey), which could alter edge-case acceptance/rejection depending on the underlying BDK version and script type handling.

Overview
validateDustLimit now uses BDK’s Amount.is_dust(account.publicAddress.script_pubkey) to enforce dust limits, removing the hardcoded per-AddressType dust threshold switch and the AddressType import.

This makes dust validation depend on the actual output script (matching Bitcoin Core relay policy) rather than a fixed table of constants.

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