lnwallet: define helper func to coop close conf scaling

We have two versions: for itests, we just use one conf, but in prod,
we'll scale the number of confirmations.

lnwallet/confscale_integration.go

@@ -0,0 +1,13 @@
+//go:build integration
+// +build integration
+
+package lnwallet
+
+import "github.com/btcsuite/btcd/btcutil"
+
+// CloseConfsForCapacity returns the number of confirmations to wait
+// before signaling a cooperative close. Under integration tests, we
+// always return 1 to keep tests fast and deterministic.
+func CloseConfsForCapacity(capacity btcutil.Amount) uint32 { //nolint:revive
+	return 1
+}

lnwallet/confscale_prod.go

@@ -0,0 +1,25 @@
+//go:build !integration
+// +build !integration
+
+package lnwallet
+
+import "github.com/btcsuite/btcd/btcutil"
+
+// CloseConfsForCapacity returns the number of confirmations to wait before
+// signaling a channel close, scaled by channel capacity. This is used for both
+// cooperative and force closes. We enforce a minimum of 3 confirmations to
+// provide better reorg protection, even for small channels.
+func CloseConfsForCapacity(capacity btcutil.Amount) uint32 {
+	// For cooperative closes, we don't have a push amount to consider,
+	// so we pass 0 for the pushAmt parameter.
+	scaledConfs := uint32(ScaleNumConfs(capacity, 0))
+
+	// Enforce a minimum of 3 confirmations for reorg safety.
+	// This protects against shallow reorgs which are more common.
+	const minCloseConfs = 3
+	if scaledConfs < minCloseConfs {
+		return minCloseConfs
+	}
+
+	return scaledConfs
+}