Add block_ref CRDT fuzz target (#1440)

Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/1440
Reviewed-by: Alex <lx@deuxfleurs.fr>

fuzz/fuzz_targets/block_ref_crdt.rs

@@ -0,0 +1,20 @@
+#![no_main]
+
+use garage_fuzz::check_crdt_laws;
+use garage_model::s3::block_ref_table::BlockRef;
+use libfuzzer_sys::fuzz_target;
+
+/// Build a BlockRef with a fixed block hash and version UUID so that CRDT state
+/// can be compared across merge results. Only the deleted flag varies.
+fn make_block_ref(deleted: bool) -> BlockRef {
+	BlockRef {
+		block: [0u8; 32].into(),
+		version: [0u8; 32].into(),
+		deleted: deleted.into(),
+	}
+}
+
+fuzz_target!(|inputs: (bool, bool, bool)| {
+	let (d1, d2, d3) = inputs;
+	check_crdt_laws(make_block_ref(d1), make_block_ref(d2), make_block_ref(d3));
+});