feat: introduce preloaded index cache and thread-safe progress tracker

Introduce `PreloadedIndex` to cache partition indices and eliminate redundant I/O during repeated queries. Refactor the query pipeline to route through this pre-loaded index, and expose it publicly in `obikpartitionner`. Additionally, add a thread-safe, lazily-initialized `MultiProgress` singleton for improved progress tracking.

src/obikindex/src/progress.rs

@@ -0,0 +1,21 @@
+use std::sync::OnceLock;
+
+use indicatif::MultiProgress;
+
+static MULTI: OnceLock<MultiProgress> = OnceLock::new();
+
+/// Initialise the shared progress display.  Call once from the binary before
+/// any index operation.  Subsequent calls are silently ignored.
+pub fn init(multi: MultiProgress) {
+    let _ = MULTI.set(multi);
+}
+
+/// Return the shared `MultiProgress`, creating a plain default one if the
+/// binary never called [`init`].
+pub fn get() -> &'static MultiProgress {
+    MULTI.get_or_init(MultiProgress::new)
+}
+
+pub(crate) fn multi() -> &'static MultiProgress {
+    get()
+}