Add public server links to README, fix dead_code warnings

- Add Free Public Servers section with US/EU endpoints and usage examples
- Add Server Pro section documenting the optional pro build
- Add Android/Termux to supported platforms and installation guide
- Gate pro-only public functions with #[cfg(feature = "pro")] to eliminate
  6 dead_code warnings in the standard build

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/bandwidth.rs

@@ -73,6 +73,7 @@ impl BandwidthState {
     }
 
     /// Set the byte budget (total bytes allowed for the entire test).
+    #[cfg(feature = "pro")]
     pub fn set_budget(&self, budget: u64) {
         self.byte_budget.store(budget, std::sync::atomic::Ordering::SeqCst);
     }