Refactor FileCache to use Condvar for efficient waits

The `FileCache` now uses `std::sync::Condvar` to signal when files have
finished loading. This replaces the previous spin-wait mechanism,
significantly improving efficiency by allowing waiting threads to sleep
until explicitly woken up.

The `wait_for_load` method now blocks on the `Condvar`, and the
`insert_m1` and `insert_tick` methods call `notify_all` after updating
the cache and removing the loading guard. This ensures that all waiting
threads are woken up and can re-check the cache.

Additionally, `ensure_m1_loaded` and `ensure_tick_loaded` in
`DataServer` have been simplified to directly call `wait_for_load` when
another thread is already loading the file, removing the redundant
synchronous loading logic.
Refactor FileCache to use Condvar

src/main.rs

@@ -5,6 +5,7 @@ use myc::ast::parser::Parser;
 use std::path::PathBuf;
 
 fn main() -> eframe::Result {
+    myc::ast::data_server::init_default_data_server();
     let options = eframe::NativeOptions {
         viewport: egui::ViewportBuilder::default().with_inner_size([1024.0, 768.0]),
         ..Default::default()