refactor(fuse): one tree for every mount

On-demand sync folders kept their own State, which the drain, the
event applier and the sweeps had to visit one by one (B74). Every mount
now serves from the primary's tree. A session keeps only its root, which
the kernel knows as inode 1, and its notification channel; work that
changes the tree tells every mounted session. Size upgrades become
daemon-wide, and moves compare mounts by root instead of by state.

crates/pdfs-fuse/src/devices.rs

@@ -27,7 +27,7 @@ use tracing::{info, warn};
 
 use super::sync::{self, base_name};
 use super::{
-    Core, SecondaryInsertRejection, SecondaryMount, State, SwitchBlocked, clear_stale_mount,
+    Core, SecondaryInsertRejection, SecondaryMount, SwitchBlocked, clear_stale_mount,
     device_type_str, dir_is_empty, evict_dir_contents, fuse_connection_id, is_stale_mount,
     now_secs, parse_uid, spawn_session, sync_folder_info, this_hostname,
 };
@@ -616,23 +616,16 @@ impl Core {
         };
     }
 
-    /// A sibling Core that shares this one's client/rt/cache/db (and transfer,
-    /// activity, mount registries) but gets a **fresh, empty `State`** — its own
-    /// inode space starting at [`ROOT_INO`]. Used to root a secondary FUSE session
-    /// at an `ondemand` sync folder without colliding with the main mount's inodes
-    /// (devices.md Phase 3).
+    /// A sibling Core for a secondary FUSE session rooted at an `ondemand` sync
+    /// folder (devices.md Phase 3). It shares everything with this one,
+    /// the tree included, except what belongs to a session: its root and its
+    /// kernel channel, both filled in when the session is built.
     pub(crate) fn fork_state(&self) -> Core {
         let mut fork = self.clone();
         fork.primary = false;
-        let share_access = self.state().share_access.clone();
-        fork.state = Arc::new(Mutex::new(State::new(self.db.clone(), share_access)));
-        // A fresh inode space needs a fresh notification channel: this fork's
-        // session is the only one that knows these inodes, so it must be the one
-        // notified about them. Filled in by `spawn_ondemand_mount`.
+        fork.root = Arc::new(OnceLock::new());
         fork.notifier = Arc::new(OnceLock::new());
         fork.session_live = Arc::new(std::sync::atomic::AtomicBool::new(false));
-        // Size upgrades are keyed by inode, which is per-fork too.
-        fork.size_upgrades = Arc::new(Mutex::new(HashMap::new()));
         fork
     }
 
@@ -934,8 +927,8 @@ impl Core {
         }
     }
 
-    /// Spawn a secondary FUSE session rooted at `root` over `local` on a forked
-    /// inode space. Clears any stale kernel mount first (a crashed run can leave
+    /// Spawn a secondary FUSE session rooted at `root` over `local` from a
+    /// [`Core::fork_state`] sibling. Clears any stale kernel mount first (a crashed run can leave
     /// one, which would fail the fresh mount with EBUSY).
     pub(crate) fn spawn_ondemand_mount(
         &self,