Complete the pinned parity example module closure

Adopt only tests/common/mod.rs from exact T835 checkpoint 29cb24a878. The already-adopted 223aa125 parity example references this missing module, preventing root cargo fmt. No in-progress product fixes or full test suites are incorporated.

crates/content-history/tests/common/mod.rs

@@ -0,0 +1,77 @@
+use content_history::{ContentId, Entry, History, Object, ObjectId, Tree};
+
+pub fn objects() -> Vec<Object> {
+    let empty = Object::Blob(Vec::new());
+    let text = Object::Blob(b"a\r\n\xc3\xa9\0".to_vec());
+    let ObjectId::Blob(empty_id) = empty.id() else {
+        unreachable!()
+    };
+    let ObjectId::Blob(text_id) = text.id() else {
+        unreachable!()
+    };
+    let entries = vec![
+        Entry {
+            key: "é".into(),
+            target: ContentId::Blob(text_id),
+        },
+        Entry {
+            key: "z".into(),
+            target: ContentId::Blob(empty_id),
+        },
+    ];
+    let map = Object::Tree(Tree::map(entries.clone()).unwrap());
+    let sequence = Object::Tree(Tree::sequence(entries).unwrap());
+    let ObjectId::Tree(root) = map.id() else {
+        unreachable!()
+    };
+    let first = Object::History(History {
+        root,
+        parent: None,
+        actor: "user:編集".into(),
+        timestamp_micros: -1,
+    });
+    let ObjectId::History(parent) = first.id() else {
+        unreachable!()
+    };
+    let returned = Object::History(History {
+        root,
+        parent: Some(parent),
+        actor: "user:編集".into(),
+        timestamp_micros: 0,
+    });
+    vec![
+        empty,
+        text,
+        Object::Tree(Tree::map(vec![]).unwrap()),
+        Object::Tree(Tree::sequence(vec![]).unwrap()),
+        map,
+        sequence,
+        first,
+        returned,
+        Object::Tree(
+            Tree::map(vec![Entry {
+                key: "nested".into(),
+                target: ContentId::Tree(root),
+            }])
+            .unwrap(),
+        ),
+    ]
+}
+
+pub fn digest_bytes(object: &Object) -> [u8; 32] {
+    match object.id() {
+        ObjectId::Blob(id) => *id.as_bytes(),
+        ObjectId::Tree(id) => *id.as_bytes(),
+        ObjectId::History(id) => *id.as_bytes(),
+    }
+}
+
+pub fn parity_word(index: u32) -> u32 {
+    let objects = objects();
+    let Some(object) = objects.get(index as usize / 8) else {
+        return u32::MAX;
+    };
+    let hash = digest_bytes(object);
+    let offset = (index as usize % 8) * 4;
+    u32::from_be_bytes(hash[offset..offset + 4].try_into().unwrap())
+}