Thread destination through max_sendable signature

Reshape MdkClient::max_sendable to accept Option<&PaymentInstructions>
so the follow-up can wire destination-aware per-hop fee inversion
(MDK-865) without churning the signature again. The None branch keeps
the v0 buffer behaviour; Some(_) trips a debug_assert until the real
implementation lands with the ldk-node bump that exposes
Node::find_route.

The daemon /getbalance handler now passes None. The JSON field
max_withdrawable_sat is unchanged.

Author: amackillop

Cargo.lock

@@ -16,7 +16,7 @@ demo = []
 
 [dependencies]
 # Branch: https://github.com/moneydevkit/ldk-node/tree/lsp-0.7.0_accept-underpaying-htlcs_with_timing_logs
-ldk-node = { git = "https://github.com/moneydevkit/ldk-node", rev = "5dce44b6e795560bbf62f49d3648308ce88a0586" }
+ldk-node = { git = "https://github.com/moneydevkit/ldk-node", rev = "f13fcead7e02ef4b77489a83854f204de11e902b" }
 
 # Pinned to the same git rev as ldk-node's transitive pull to avoid duplicate
 # crate compilation. Verify with `cargo tree -d | grep bitcoin-payment-instructions`.

Cargo.toml

@@ -35,7 +35,7 @@ pub async fn handle_get_balance(
         .map(|ch| ch.outbound_capacity_msat / 1000)
         .sum();
 
-    let max_withdrawable_sat = client.max_sendable().ok().map(|e| e.amount_msat / 1000);
+    let max_withdrawable_sat = client.max_sendable(None).ok().map(|e| e.amount_msat / 1000);
 
     Ok(Json(GetBalanceResponse {
         balance_sat: lightning_sat,

src/daemon/api/balance.rs

@@ -1,6 +1,7 @@
 use std::str::FromStr;
 use std::sync::Arc;
 
+use bitcoin_payment_instructions::PaymentInstructions;
 use chrono::{DateTime, SecondsFormat};
 use ldk_node::bitcoin::hashes::sha256;
 use ldk_node::bitcoin::hashes::Hash as _;
@@ -147,7 +148,18 @@ impl MdkClient {
     /// over Lightning right now, with routing-fee headroom subtracted.
     /// Computed inline from `node.list_channels()` on every call so
     /// the result reflects in-flight HTLCs and reserve as of *now*.
-    pub fn max_sendable(&self) -> Result<MaxSendableEstimate, MaxSendableError> {
+    ///
+    /// `dest` is currently ignored — only the `None` (destination-agnostic
+    /// buffer) path is implemented.
+    pub fn max_sendable(
+        &self,
+        dest: Option<&PaymentInstructions>,
+    ) -> Result<MaxSendableEstimate, MaxSendableError> {
+        debug_assert!(
+            dest.is_none(),
+            "destination-aware max_sendable is not yet implemented"
+        );
+        let _ = dest;
         let snaps: Vec<ChannelSnapshot> = self
             .node
             .list_channels()