Add TF listener/faker examples and cross-compile support

Add two new examples demonstrating TF (transform) message handling with
rosrust. The tf_listener example subscribes to transform messages from
a ROS master and prints them to stdout. The tf_faker example publishes
synthetic transform data in a circular motion pattern.

Add Cross.toml configuration to enable cross-compilation by passing
through the ROSRUST_MSG_PATH environment variable, which is required
for message code generation during cross-compilation to different
architectures like arm64.

Update README with documentation for both new examples and simplify
cross-compilation instructions to use the cross tool instead of manual
environment variable setup. This reduces friction for developers
targeting non-native platforms.

Author: PatWie

Cross.toml

@@ -0,0 +1,2 @@
+[build.env]
+passthrough = ["ROSRUST_MSG_PATH"]

README.md

@@ -43,6 +43,28 @@ implementation is inspired by the official chatter python example from the ROS
 wiki, which demonstrates a simple communication between two nodes using ROS
 messages.
 
+### TF Listener/Faker
+
+Subscribe to TF messages from an external ROS master:
+
+```bash
+RUST_LOG=info ROSRUST_MSG_PATH=`realpath messages` cargo run --example tf_listener
+```
+
+Publish fake TF transforms:
+
+```bash
+RUST_LOG=info ROSRUST_MSG_PATH=`realpath messages` cargo run --example tf_faker
+```
+
+Use `ROS_MASTER_URI` to connect to a different master (default: `http://localhost:11311`).
+
+Cross-compile for arm64:
+
+```bash
+ROSRUST_MSG_PATH=`realpath messages` cross build --example tf_listener --target aarch64-unknown-linux-gnu --release
+```
+
 ### Debugging with official ROS docker image
 
 To showcase that this ROS core implementation can be used with official ROS
@@ -74,9 +96,7 @@ contribute code, feel free to submit a pull request.
 
 
 ## Cross-compilation to arm64
+
 ```bash
-apt install libssl-dev:arm64
-export AARCH64_UNKNOWN_LINUX_GNU_OPENSSL_LIB_DIR=/usr/lib/aarch64-linux-gnu/
-export AARCH64_UNKNOWN_LINUX_GNU_OPENSSL_INCLUDE_DIR=/usr/include/aarch64-linux-gnu/
-cargo build --target aarch64-unknown-linux-gnu --release
+cross build --target aarch64-unknown-linux-gnu --release
 ```

examples/tf_faker.rs

@@ -0,0 +1,37 @@
+use std::env;
+
+fn main() {
+    env_logger::init();
+
+    let master_uri = env::var("ROS_MASTER_URI").unwrap_or_else(|_| "http://localhost:11311".into());
+    env::set_var("ROS_MASTER_URI", &master_uri);
+
+    rosrust::init("tf_faker");
+
+    let pub_tf = rosrust::publish("/tf", 10).expect("Failed to create /tf publisher");
+    let rate = rosrust::rate(10.0);
+    let mut seq = 0u32;
+
+    while rosrust::is_ok() {
+        let now = rosrust::now();
+        let t = seq as f64 * 0.1;
+
+        let mut msg = rosrust_msg::tf2_msgs::TFMessage::default();
+        let mut tf = rosrust_msg::geometry_msgs::TransformStamped::default();
+
+        tf.header.seq = seq;
+        tf.header.stamp = now;
+        tf.header.frame_id = "world".into();
+        tf.child_frame_id = "base_link".into();
+        tf.transform.translation.x = t.sin();
+        tf.transform.translation.y = t.cos();
+        tf.transform.translation.z = 0.0;
+        tf.transform.rotation.w = 1.0;
+
+        msg.transforms.push(tf);
+        pub_tf.send(msg).ok();
+
+        seq += 1;
+        rate.sleep();
+    }
+}

examples/tf_listener.rs

@@ -0,0 +1,30 @@
+use std::env;
+
+fn main() {
+    env_logger::init();
+
+    let master_uri = env::var("ROS_MASTER_URI").unwrap_or_else(|_| "http://localhost:11311".into());
+    env::set_var("ROS_MASTER_URI", &master_uri);
+
+    rosrust::init("tf_listener");
+
+    let _sub = rosrust::subscribe("/tf", 10, |msg: rosrust_msg::tf2_msgs::TFMessage| {
+        for tf in &msg.transforms {
+            println!(
+                "{} -> {}: t=[{:.3}, {:.3}, {:.3}] r=[{:.3}, {:.3}, {:.3}, {:.3}]",
+                tf.header.frame_id,
+                tf.child_frame_id,
+                tf.transform.translation.x,
+                tf.transform.translation.y,
+                tf.transform.translation.z,
+                tf.transform.rotation.x,
+                tf.transform.rotation.y,
+                tf.transform.rotation.z,
+                tf.transform.rotation.w,
+            );
+        }
+    })
+    .expect("Failed to subscribe to /tf");
+
+    rosrust::spin();
+}