Phase 3a: Lamport-stamped refs with CRDT-merge on PutRef

Concurrent PutRef safety via (clock, node) total order. New wire
methods PutRefVersioned (0x15) + GetRefVersioned (0x16). Existing
PutRef/GetRef unchanged for backward compat. Data in refs-v2/
namespace so the two coexist during cutover.

+8 tests, 272 total (baseline +8).

claw-store/src/cluster/rpc/client.rs

@@ -180,6 +180,7 @@ pub fn decode_error(b: u8) -> Option<ErrorCode> {
         0xf3 => Some(ErrorCode::NotFound),
         0xf4 => Some(ErrorCode::InvalidRequest),
         0xf5 => Some(ErrorCode::NotConfigured),
+        0xf6 => Some(ErrorCode::AlreadyExists),
         _ => None,
     }
 }
@@ -472,6 +473,61 @@ pub async fn call_put_ref(
     }
 }
 
+// ── Phase 3: stamped-ref client helpers ──────────────────────────────
+
+/// Phase 3 (2026-07-13): submit a stamped (CRDT-merge) PutRef.
+///
+/// Returns `Ok(true)` when the peer merged the write (Merged),
+/// `Ok(false)` when the peer rejected it because an equal or
+/// higher `(clock, node)` already exists (AlreadyExists). Any
+/// other reply is an error.
+pub async fn call_put_ref_versioned(
+    conn: &Connection,
+    key: &crate::cluster::refs::RefKey,
+    incoming: &crate::cluster::refs::StampedRef,
+) -> Result<bool> {
+    let mut payload = Vec::with_capacity(32 + crate::cluster::refs::StampedRef::ENCODED_LEN);
+    payload.extend_from_slice(key);
+    payload.extend_from_slice(&incoming.to_bytes());
+    let reply = rpc_call(conn, Method::PutRefVersioned, &payload).await?;
+    if reply.len() != 1 {
+        bail!(
+            "expected single-byte PutRefVersioned reply, got {} bytes",
+            reply.len()
+        );
+    }
+    match reply[0] {
+        STREAM_STATUS_OK => Ok(true),
+        code => match decode_error(code) {
+            Some(ErrorCode::AlreadyExists) => Ok(false),
+            Some(err) => bail!("peer rejected PutRefVersioned: {}", err.describe()),
+            None => bail!(
+                "peer replied with unknown byte 0x{:02x} for PutRefVersioned",
+                code
+            ),
+        },
+    }
+}
+
+/// Phase 3: fetch a stamped (CRDT-merge) ref.
+pub async fn call_get_ref_versioned(
+    conn: &Connection,
+    key: &crate::cluster::refs::RefKey,
+) -> Result<Option<crate::cluster::refs::StampedRef>> {
+    let reply = rpc_call(conn, Method::GetRefVersioned, key).await?;
+    if reply.len() == 1 {
+        match decode_error(reply[0]) {
+            Some(ErrorCode::NotFound) => return Ok(None),
+            Some(err) => bail!("peer replied with error: {}", err.describe()),
+            None => {}
+        }
+    }
+    Ok(Some(
+        crate::cluster::refs::StampedRef::from_bytes(&reply)
+            .context("decoding stamped ref reply")?,
+    ))
+}
+
 // ── Phase 5g: cache metrics client helper ────────────────────────────
 
 /// Fetch the peer's current cache-metrics snapshot.