fix(fuse,core): renames across folders, replaced files, and writes after a create

Found by the seeded simulation runs (docs/BUGS.md).

- B122: a rename across folders goes to Drive as one move-multiple
  request, so a sibling holding the new name in the old folder no longer
  fails it with EIO. The drain uses the same request.
- B123: a second write open on a file just written no longer fails with
  EIO. An empty file counts as complete, our own renames, moves and
  online creates record their echo, a duplicated event is skipped, a
  rename keeps the file's mtime while its revision stays, and a landed
  upload is cached before open handles are rebased onto it.
- B124: a node put back after a failed replacing rename leaves `hidden`
  and its folder is listed again, so it shows in the mount.
- B131: a landed create is read back with the same short retry as an
  online create, and its bytes are cached under the real uid before an
  open handle is repointed at it.
- B135: an online rename settles the tree through the state guard, so
  the database has both folders unlisted before the reply. A lookup
  right after it no longer rebuilds the destination without the file.

The simulation runs no longer tolerate B122 or B131.

crates/pdfs-core/src/cache.rs

@@ -365,6 +365,14 @@ pub struct StagedWrite {
     pub based_on: Option<Baseline>,
 }
 
+impl StagedWrite {
+    /// Whether `authored` ranges cover a file of `len` bytes, so the staged
+    /// bytes are its complete new content. An empty file has nothing to cover.
+    pub fn covers(authored: &[(u64, u64)], len: u64) -> bool {
+        len == 0 || authored == [(0, len)]
+    }
+}
+
 /// The identity of a remote revision, as far as we can observe it.
 ///
 /// `revision_id` is the authoritative identity when present: the server assigns a
@@ -2056,6 +2064,17 @@ mod tests {
         NodeUid::new(VolumeId::from("vol"), LinkId::from(link))
     }
 
+    #[test]
+    fn an_empty_write_covers_the_file_and_a_gap_does_not() {
+        assert!(
+            StagedWrite::covers(&[], 0),
+            "an empty file has nothing to cover"
+        );
+        assert!(StagedWrite::covers(&[(0, 7)], 7));
+        assert!(!StagedWrite::covers(&[(0, 3), (4, 7)], 7));
+        assert!(!StagedWrite::covers(&[], 7));
+    }
+
     /// A unique temp directory removed on drop; avoids a dev-dependency.
     struct TempDir(PathBuf);
     impl TempDir {