proof: add FFM proof for call-c-from-java pattern (#129)

Uses strlen from the system C library via the Foreign Function &
Memory API to prove the FFM code compiles and runs correctly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brunoborges

proof/language/CallCFromJava.java

@@ -0,0 +1,25 @@
+///usr/bin/env jbang "$0" "$@" ; exit $?
+//JAVA 25+
+//JAVA_OPTIONS --enable-native-access=ALL-UNNAMED
+
+import java.lang.foreign.*;
+import java.lang.invoke.MethodHandle;
+import java.util.Optional;
+
+/// Proof: call-c-from-java
+/// Source: content/language/call-c-from-java.yaml
+void main() throws Throwable {
+
+    try (Arena arena = Arena.ofConfined()) {
+        // Use a system library to prove FFM compiles and links
+        SymbolLookup stdlib = Linker.nativeLinker().defaultLookup();
+        Optional<MemorySegment> segment = stdlib.find("strlen");
+        MemorySegment foreignFuncAddr = segment.get();
+        FunctionDescriptor strlen_sig =
+            FunctionDescriptor.of(ValueLayout.JAVA_LONG, ValueLayout.ADDRESS);
+        MethodHandle strlenMethod =
+            Linker.nativeLinker().downcallHandle(foreignFuncAddr, strlen_sig);
+        var ret = (long) strlenMethod.invokeExact(arena.allocateFrom("Bambi"));
+        System.out.println("Return value " + ret);
+    }
+}