chore(firmware): stamp builds with UTC time and git hash

Emit BUILD_TIME and BUILD_GIT (git describe --always --dirty) as
rustc env vars so the running firmware can identify itself on serial
and on-panel. A rerun-if-changed on a nonexistent file forces the
script every build, keeping the timestamp fresh. Bring-up lesson:
know which build you're diagnosing.

firmware/build.rs

@@ -1,3 +1,27 @@
+use std::process::Command;
+
 fn main() {
     embuild::espidf::sysenv::output();
+
+    // Stamp the binary so serial output and on-panel text identify the
+    // exact build (bring-up lesson: know which build you're diagnosing).
+    let git = Command::new("git")
+        .args(["describe", "--always", "--dirty"])
+        .output()
+        .ok()
+        .filter(|o| o.status.success())
+        .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string())
+        .unwrap_or_else(|| "unknown".into());
+    let time = Command::new("date")
+        .args(["-u", "+%m-%d %H:%M"])
+        .output()
+        .ok()
+        .filter(|o| o.status.success())
+        .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string())
+        .unwrap_or_else(|| "unknown".into());
+    println!("cargo:rustc-env=BUILD_GIT={git}");
+    println!("cargo:rustc-env=BUILD_TIME={time}Z");
+    // Pointing rerun-if-changed at a file that never exists forces this
+    // script to rerun on every build, keeping BUILD_TIME fresh.
+    println!("cargo:rerun-if-changed=.force-build-stamp");
 }