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

AGENTS.md

@@ -6,16 +6,17 @@ This project is a small Rust common library and runtime built without Cargo.
 CMake invokes `rustc` directly. The library provides a low-footprint subset
 of standard-library-like functionality:
 
-- `Result` and formatting from `core`
-- `Vec` and `vec![]` from `alloc`
+- `Result`, `Duration`, and formatting from `core`
+- `Vec`, `String`, `vec![]`, and `format!` from `alloc`
 - project-local `print!` and `println!` macros
-- direct platform stdout and heap calls
+- direct platform stdout, heap calls, and busy-wait sleep
 
 The project contains:
 
 - `src/lib.rs`: the `#![no_std]` common library API
 - `src/io.rs`: the `core::fmt::Write` stdout backend
-- `src/runtime.rs`: allocator, async polling, and platform entry helpers
+- `src/runtime.rs`: allocator, async polling, monotonic sleep, and platform
+  entry helpers
 - `macros/src/lib.rs`: host-side `#[rslibc_macros::main]` attribute macro
 - `runtime/main.rs`: minimal example entry point
 - `CMakeLists.txt`: direct `rustc` library, macro, and executable commands
@@ -36,8 +37,8 @@ The project contains:
 - Keep OS integration in small FFI modules. Do not pull in a general-purpose
   runtime or C I/O layer for basic output.
 - `rslibc_macros::main` can wrap `async fn main()` with the small `block_on`
-  executor. It is only a polling loop and does not provide Tokio-style I/O,
-  timers, or a reactor.
+  executor. It is only a polling loop and does not provide Tokio-style I/O
+  or a reactor; delays use the busy-wait `rslibc::runtime::sleep`.
 - Keep unsafe code inside small safe wrappers. Application code should be
   able to use calls such as:
 
@@ -52,6 +53,8 @@ The project contains:
 - Windows stdout calls `GetStdHandle` and `WriteFile` through `kernel32`.
 - Windows allocation calls `HeapAlloc` and `HeapFree` through the system
   heap.
+- Sleep uses a monotonic clock: `clock_gettime` on Unix and
+  `QueryPerformanceCounter` on Windows.
 - The Windows runtime dynamically links only the platform support needed by
   `alloc` and the compiler runtime. Do not statically link the full C
   runtime.
@@ -116,7 +119,8 @@ cmake --build build
 ```
 
 On Windows, use `Remove-Item -Recurse -Force build` and run
-`build\\rslibc_example.exe` instead. Confirm that stdout contains `Hello`.
+`build\\rslibc_example.exe` instead. Confirm that stdout contains
+`100% done`.
 
 When size matters, inspect the Windows executable after a release build.
 Avoid adding formatting, allocation, or OS dependencies unless they are