fix: set is_warmup_completed true when warmup/indexing disabled (#451)

Refs #451

When both enable_mmap_cache and enable_content_indexing are false,
run_post_scan never executes (guarded by config.warmup || config.content_indexing).
This left bigram_index as None, so get_scan_progress().is_warmup_complete
returned false forever.

Added warmup_complete flag to ScanSignals. Set immediately after post-scan
finishes OR right after walking completes when no post-scan work needed.
get_scan_progress now reads warmup_complete instead of checking bigram_index
presence.

crates/fff-core/src/file_picker.rs

@@ -1210,11 +1210,13 @@ impl FilePicker {
     pub fn get_scan_progress(&self) -> ScanProgress {
         let scanned_count = self.scanned_files_count.load(Ordering::Relaxed);
         let is_scanning = self.signals.scanning.load(Ordering::Relaxed);
+
         ScanProgress {
             scanned_files_count: scanned_count,
             is_scanning,
             is_watcher_ready: self.signals.watcher_ready.load(Ordering::Relaxed),
-            is_warmup_complete: self.sync_data.bigram_index.is_some(),
+            is_warmup_complete: !self.enable_content_indexing
+                || self.sync_data.bigram_index.is_some(),
         }
     }