feat: add loading animation example

Add a busy-wait sleep on a monotonic clock and re-export String,
format!, and Duration from the library. The example animates a
progress bar with a spinner and logs each step in a Vec<String>.

Co-Authored-By: qwen (lmstudio/qwen3.8-27b@q3_k_xl): wrote example

README.md

@@ -8,7 +8,8 @@ library rlib, and a minimal example executable.
 
 - `Result` and formatting from `core`
 - `Vec` and `vec![]` from `alloc`
-- project-local `print!` / `println!` macros
+- project-local `print!` / `println!` macros, plus `String` and `format!`
+- busy-wait `sleep` on a monotonic clock
 - direct platform stdout with no C I/O layer
 - OS heap allocator (`malloc`/`free`, `HeapAlloc`/`HeapFree`)
 - `panic=abort` with a minimal panic handler
@@ -41,8 +42,9 @@ Run the example:
 .\build\rslibc_example.exe
 ```
 
-The release example prints `Hello, World!`. The Windows executable is
-about 4 KiB and depends only on `KERNEL32.dll` and `VCRUNTIME140.dll`.
+The release example runs an animated loading loop (progress bar plus
+spinner) and then prints a short log. The Windows executable is about
+27 KiB and depends only on `KERNEL32.dll` and `VCRUNTIME140.dll`.
 
 Individual targets: