fix(engine): close the Windows ledger and sidecar save gaps

- FleetLedger caches only the durable length, not an open ledger handle,
  so a live appender no longer blocks compaction's replace on Windows.
- hash_session_record treats a Windows path-not-found under a
  non-directory parent as NotADirectory, matching Unix, so a blocked
  checkpoint inspection cannot silently clear save health.
- The focused Windows workflow now carries RUST_BACKTRACE for Phase B.

Local: persistence_actor 1/1, ledger compaction 1/1, fleet_setup 1/1;
scripts/preflight.sh --check OK.

.github/workflows/windows-focus.yml

@@ -26,6 +26,7 @@ jobs:
       CARGO_INCREMENTAL: 0
       RUSTFLAGS: -Dwarnings
       RUST_MIN_STACK: 8388608
+      RUST_BACKTRACE: full
       FILTER: ${{ inputs.filter }}
     steps:
       - uses: actions/checkout@v7

crates/engine/src/fleet/ledger.rs

@@ -342,7 +342,7 @@ pub struct FleetLedger {
     ledger_file: WorkspaceFile,
     lock_file: WorkspaceFile,
     original_lock: std::fs::File,
-    durable_barrier: parking_lot::Mutex<Option<(std::fs::File, u64)>>,
+    durable_barrier: parking_lot::Mutex<Option<u64>>,
     /// Stable advisory-lock inode shared by every manager for this workspace.
     ///
     /// The ledger itself is replaced during compaction, so locking
@@ -549,7 +549,7 @@ impl FleetLedger {
         file.sync_data()
             .with_context(|| format!("syncing fleet ledger {}", self.ledger_path.display()))?;
         let length = file.metadata()?.len();
-        *self.durable_barrier.lock() = Some((file, length));
+        *self.durable_barrier.lock() = Some(length);
         // Wake SSE subscribers after the bytes are durable (#6211 R7b). Every
         // record kind funnels through here, so a wake only promises "something
         // changed" — subscribers re-poll with their cursor.
@@ -1186,17 +1186,12 @@ impl FleetLedger {
         let file = self.ledger_file.open_update(false, false)?;
         let length = file.metadata()?.len();
         let mut cached = self.durable_barrier.lock();
-        if let Some((previous, durable_length)) = cached.take()
-            && durable_length == length
-            && same_file(&previous, &file)?
-        {
-            *cached = Some((previous, durable_length));
+        if *cached == Some(length) {
             return Ok(());
         }
         file.sync_data()
             .context("acknowledging Fleet ledger durability")?;
-        let length = file.metadata()?.len();
-        *cached = Some((file, length));
+        *cached = Some(file.metadata()?.len());
         Ok(())
     }
 

crates/engine/src/session_manager.rs

@@ -3962,7 +3962,21 @@ impl SessionManager {
         use std::io::Read;
         let mut file = match open_private_read_file(path) {
             Ok(file) => file,
-            Err(error) if error.kind() == io::ErrorKind::NotFound => return Ok(None),
+            Err(error) if error.kind() == io::ErrorKind::NotFound => {
+                if path
+                    .parent()
+                    .is_some_and(|parent| parent.exists() && !parent.is_dir())
+                {
+                    return Err(io::Error::new(
+                        io::ErrorKind::NotADirectory,
+                        format!(
+                            "private sidecar parent {} is not a directory",
+                            path.parent().unwrap_or(path).display()
+                        ),
+                    ));
+                }
+                return Ok(None);
+            }
             Err(error) => return Err(error),
         };
         let mut digest = Sha256::new();