feat: --version flag with git hash + test script kill fix

wzp-relay --version prints "wzp-relay <short-git-hash>".
Build hash also logged on startup: version=abc1234.
Enables verifying deployed relay matches expected build.

Also fixed federation-test.sh: use kill -INT (not SIGTERM) so
clients save recordings before exit. Added save delay.

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

crates/wzp-relay/build.rs

@@ -0,0 +1,18 @@
+use std::process::Command;
+
+fn main() {
+    // Get git hash at build time
+    let output = Command::new("git")
+        .args(["rev-parse", "--short", "HEAD"])
+        .output();
+
+    let hash = match output {
+        Ok(o) if o.status.success() => {
+            String::from_utf8_lossy(&o.stdout).trim().to_string()
+        }
+        _ => "unknown".to_string(),
+    };
+
+    println!("cargo:rustc-env=WZP_BUILD_HASH={hash}");
+    println!("cargo:rerun-if-changed=.git/HEAD");
+}