test: provide native signal book identity probe

crates/fidc-core/examples/signal_book_probe.rs

@@ -0,0 +1,13 @@
+use std::io::{self,Read};
+
+fn main() -> Result<(),Box<dyn std::error::Error>> {
+    let mut input=String::new();
+    io::stdin().read_to_string(&mut input)?;
+    let book:fidc_core::signal_contract::SignalBook=serde_json::from_str(&input)?;
+    let version=book.version_sha256.clone();
+    let snapshots=book.snapshots.len();
+    let validated=book.validate().map_err(io::Error::other)?;
+    println!("{}",serde_json::json!({"ok":true,"versionSha256":version,"snapshots":snapshots,
+        "symbols":validated.symbols().len(),"observed":validated.require_observed().is_ok()}));
+    Ok(())
+}