feat(key-wallet): widen DEFAULT_COINJOIN_GAP_LIMIT 30 -> 100 for dashj parity (proposal)

key-wallet/src/gap_limit.rs

@@ -16,8 +16,31 @@ pub const DEFAULT_EXTERNAL_GAP_LIMIT: u32 = 30;
 /// Standard gap limit for internal (change) addresses
 pub const DEFAULT_INTERNAL_GAP_LIMIT: u32 = 30;
 
-/// Standard gap limit for CoinJoin addresses
-pub const DEFAULT_COINJOIN_GAP_LIMIT: u32 = 30;
+/// Standard gap limit for CoinJoin addresses.
+///
+/// Set to match dashj's `DeterministicKeyChain` lookahead
+/// (`DEFAULT_LOOKAHEAD_SIZE = 100`), which is the effective discovery window
+/// dashj-core (the reference wallet, `org.dashj:dashj-core:22.x`) watches for
+/// CoinJoin keychains. Address discovery here is dynamic — the pool watches
+/// `highest_used + gap_limit` addresses — so this bounds the largest run of
+/// UNUSED CoinJoin addresses the wallet can bridge during a filter/rescan
+/// before discovery stalls.
+///
+/// The previous value of 30 starved heavy mixers: DIP-9 CoinJoin mixing
+/// sprays denominations across many sequential addresses, and a run wider
+/// than 30 consecutive unused addresses caused the SDK to stop discovering —
+/// it then missed both the txs that CREATED far-index UTXOs and the txs that
+/// SPENT nearer ones, leaving a persistent balance mismatch vs dashj that
+/// survived a clean re-creation + full rescan (dashpay/platform#4074,
+/// dashpay/dash-wallet#1507). 100 restores parity with dashj's window so the
+/// SDK discovers every CoinJoin tx dashj does.
+///
+/// Cost is modest: the CoinJoin account watches two pools (external +
+/// internal), so this pre-derives and filter-matches ~200 CoinJoin addresses
+/// per account instead of ~60. BIP158 filter matching is a set intersection,
+/// so the extra scripts add negligible per-block work; the derivation is a
+/// one-time keychain expansion.
+pub const DEFAULT_COINJOIN_GAP_LIMIT: u32 = 100;
 
 /// Standard gap limit for special purpose keys (identity, provider keys)
 pub const DEFAULT_SPECIAL_GAP_LIMIT: u32 = 5;