perf: Chunk-based prefiltering (+50% faster on small queries)

Author: dmtrKovalenko

Cargo.lock

@@ -639,6 +639,52 @@ pub unsafe extern "C" fn fff_wait_for_scan(
     FffResult::ok_int(1)
 }
 
+/// Wait for the background file watcher to be ready.
+///
+/// The watcher is created after the initial scan, git status, and optional
+/// warmup phases complete. Returns 1 if the watcher is ready, 0 if timed out.
+///
+/// ## Safety
+/// `fff_handle` must be a valid instance pointer from `fff_create_instance`.
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn fff_wait_for_watcher(
+    fff_handle: *mut c_void,
+    timeout_ms: u64,
+) -> *mut FffResult {
+    let inst = match unsafe { instance_ref(fff_handle) } {
+        Ok(i) => i,
+        Err(e) => return e,
+    };
+
+    let watcher_signal = {
+        let guard = match inst.picker.read() {
+            Ok(g) => g,
+            Err(e) => return FffResult::err(&format!("Failed to acquire file picker lock: {}", e)),
+        };
+
+        let picker = match guard.as_ref() {
+            Some(p) => p,
+            None => return FffResult::err("File picker not initialized"),
+        };
+
+        picker.watcher_signal()
+    };
+
+    let timeout = Duration::from_millis(timeout_ms);
+    let start = std::time::Instant::now();
+    let mut sleep_duration = Duration::from_millis(1);
+
+    while !watcher_signal.load(std::sync::atomic::Ordering::Acquire) {
+        if start.elapsed() >= timeout {
+            return FffResult::ok_int(0);
+        }
+        std::thread::sleep(sleep_duration);
+        sleep_duration = std::cmp::min(sleep_duration * 2, Duration::from_millis(50));
+    }
+
+    FffResult::ok_int(1)
+}
+
 /// Restart indexing in a new directory.
 ///
 /// ## Safety