test(chain): add MTP integration test for canonicalize_with_mtp

Covers the previously-untested median-time-past path: a `LocalChain<Header>`
whose block times equal their heights (so the MTP of an 11-block window is
the middle height), with a tx confirmed at a known height. Asserts both the
tip MTP and the per-tx MTP.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: evanlinjin

crates/chain/tests/test_canonical_view_mtp.rs

@@ -0,0 +1,86 @@
+#![cfg(feature = "miniscript")]
+
+//! Integration test for median-time-past (MTP) computation via
+//! [`LocalChain::canonicalize_with_mtp`].
+
+use std::collections::BTreeMap;
+
+use bdk_chain::{local_chain::LocalChain, ChainPosition, ConfirmationBlockTime, TxGraph};
+use bdk_testenv::{hash, utils::new_tx};
+use bitcoin::{
+    block::Header, hashes::Hash, Amount, BlockHash, OutPoint, ScriptBuf, Transaction, TxIn, TxOut,
+};
+
+/// Build a block header with an explicit `time`.
+fn header(prev_blockhash: BlockHash, time: u32) -> Header {
+    Header {
+        version: bitcoin::block::Version::default(),
+        prev_blockhash,
+        merkle_root: bitcoin::hash_types::TxMerkleNode::all_zeros(),
+        time,
+        bits: bitcoin::CompactTarget::default(),
+        nonce: 0,
+    }
+}
+
+/// Build a connected chain of blocks `0..=tip_height` where each block's timestamp equals its
+/// height (the block at height `h` has `time == h`).
+///
+/// With timestamps equal to heights, the median-time-past at height `h` — the median of the 11
+/// timestamps in `h-10..=h` — is simply the middle height, `h - 5`.
+fn chain_with_time_equal_to_height(tip_height: u32) -> LocalChain<Header> {
+    let mut headers = vec![header(BlockHash::all_zeros(), 0)]; // genesis at height 0
+    for h in 1..=tip_height {
+        let prev = headers[(h - 1) as usize].block_hash();
+        headers.push(header(prev, h));
+    }
+    let blocks: BTreeMap<u32, Header> = headers
+        .into_iter()
+        .enumerate()
+        .map(|(height, hdr)| (height as u32, hdr))
+        .collect();
+    LocalChain::from_blocks(blocks).expect("chain connects from genesis")
+}
+
+#[test]
+fn canonicalize_with_mtp_computes_median_time_past() {
+    // A 13-block chain (heights 0..=12) where each block's time equals its height.
+    let chain = chain_with_time_equal_to_height(12);
+
+    // A single transaction confirmed at height 11.
+    let mut tx_graph = TxGraph::default();
+    let tx = Transaction {
+        input: vec![TxIn {
+            previous_output: OutPoint::new(hash!("parent"), 0),
+            ..Default::default()
+        }],
+        output: vec![TxOut {
+            value: Amount::from_sat(50_000),
+            script_pubkey: ScriptBuf::new(),
+        }],
+        ..new_tx(1)
+    };
+    let txid = tx.compute_txid();
+    let _ = tx_graph.insert_tx(tx);
+    let _ = tx_graph.insert_anchor(
+        txid,
+        ConfirmationBlockTime {
+            block_id: chain.get(11).unwrap().block_id(),
+            confirmation_time: 0,
+        },
+    );
+
+    // Canonicalize with MTP enabled.
+    let view = chain.canonicalize_with_mtp(&tx_graph, chain.tip().block_id(), Default::default());
+
+    // Tip is at height 12, so its MTP is the median of heights 2..=12, which is 7.
+    assert_eq!(view.tip_mtp(), Some(7));
+
+    // The transaction is confirmed at height 11, so its MTP is the median of heights 1..=11 == 6.
+    let canonical_tx = view
+        .txs()
+        .find(|c| c.txid == txid)
+        .expect("tx is canonical");
+    assert!(matches!(canonical_tx.pos, ChainPosition::Confirmed { .. }));
+    assert_eq!(canonical_tx.mtp, Some(6));
+}