Add a global dust filter setting, idealy configurable amount

[matthiasdebernardini]

Add a configurable dust threshold to Wallet that filters outputs below a specified satoshi value from balance calculations, sent_and_received(), and UTXO iteration methods.

Currently, to exclude dust outputs (e.g., < 1000 sats) from wallet operations, users must:

1. Manually iterate list_unspent() and filter by value
2. Reimplement BDK's internal logic for categorizing UTXOs into confirmed, trusted_pending, untrusted_pending, and immature
3. Manually calculate sent_and_received() by iterating transaction inputs/outputs and looking up previous outputs

This is error-prone and requires duplicating internal BDK logic that may change between versions.

Proposed API

// Option 1: Builder pattern
let wallet = Wallet::builder()
.descriptor(descriptor)
.change_descriptor(change_descriptor)
.dust_threshold(Amount::from_sat(1000))
.network(Network::Bitcoin)
.build()?;

// Option 2: Runtime configuration
wallet.set_dust_threshold(Some(Amount::from_sat(1000)));

// These would now respect the threshold:
let balance = wallet.balance(); // Excludes UTXOs < 1000 sats
let (sent, received) = wallet.sent_and_received(&tx); // Excludes dust outputs
let utxos = wallet.list_unspent(); // Excludes dust UTXOs

Use case

We operate a multi-sig custody platform where dust outputs create operational overhead:

• Users see confusing small amounts in their balance
• Transaction history shows spam/dust attack outputs
• Consolidating dust costs more in fees than the outputs are worth
• Affects user privacy by triggering the common ownership heuristic

We need consistent dust filtering across all wallet operations. Currently we're forced to bypass wallet.balance() and sent_and_received() entirely, manually reimplementing the UTXO categorization logic.

Impact

• Nice-to-have / UX improvement
• Developer experience / maintainability

Are you using BDK in a production project?

• Yes

Which backend(s) are relevant (if any)?

• Electrum
• Esplora
• Bitcoin Core RPC

Project or organization (optional)

AnchorWatch - Bitcoin custody platform

Additional context

The 546 sat dust limit is a relay policy, but many applications want higher thresholds (1000+ sats) for UX reasons. A configurable threshold at the wallet level would be more ergonomic than forcing every consumer
to reimplement filtering logic.