refactor(core): WallClockStore trait replaces the store_mut backdoor

Task 10b reached LogStore::set_wall_clock through a general Node::store_mut.
Kyle: replace it with a narrower accessor. A WallClockStore trait (set_wall_clock
+ peer_time_estimate, implemented only by LogStore) now backs Node::set_wall_clock
and Node::peer_time_estimate, so no &mut Store escapes the facade.

tines-store/src/log.rs

@@ -88,7 +88,7 @@ use std::io::Write;
 use std::path::{Path, PathBuf};
 
 use serde::{Deserialize, Serialize};
-use tines_core::{Observation, ObservationId, Store, StoreError, Tick, TrimPolicy};
+use tines_core::{Observation, ObservationId, Store, StoreError, Tick, TrimPolicy, WallClockStore};
 
 mod crc32c {
     //! A minimal, std-only CRC-32C (Castagnoli).
@@ -802,6 +802,18 @@ impl LogStore {
     }
 }
 
+impl WallClockStore for LogStore {
+    type FoldReport = FoldReport;
+
+    fn set_wall_clock(&mut self, wall_now: u64, now: Tick) -> FoldReport {
+        LogStore::set_wall_clock(self, wall_now, now)
+    }
+
+    fn peer_time_estimate(&self, now: Tick) -> Option<u64> {
+        LogStore::peer_time_estimate(self, now)
+    }
+}
+
 /// Write the log header: `magic ‖ format_version ‖ reserved`.
 fn write_header(file: &mut File) -> Result<(), StoreError> {
     file.write_all(&MAGIC).map_err(|_| StoreError::Backend)?;