feat(scrub): dispatch EC CHECKSUM (mode 4) to checksum_scrub

Accept VolumeScrubMode.CHECKSUM=4 and route it to EcVolume::checksum_scrub,
accumulating blocks scanned + mismatched shards into the scrub response, plus the
CHECKSUM scrub-mode metric label. Read-only bitrot verification over local shards.

Claude-Session: https://claude.ai/code/session_015EE9Sc9EvNp8BCVva4RKdo

seaweed-volume/src/server/grpc_server.rs

@@ -37,6 +37,7 @@ fn scrub_mode_label(mode: i32) -> &'static str {
         1 => "INDEX",
         2 => "FULL",
         3 => "LOCAL",
+        4 => "CHECKSUM",
         _ => "UNKNOWN",
     }
 }
@@ -3911,7 +3912,7 @@ impl VolumeServer for VolumeGrpcService {
         // Validate mode
         let mode = req.mode;
         match mode {
-            1 | 2 | 3 => {} // INDEX=1, FULL=2, LOCAL=3
+            1 | 2 | 3 | 4 => {} // INDEX=1, FULL=2, LOCAL=3, CHECKSUM=4
             _ => {
                 return Err(Status::invalid_argument(format!(
                     "unsupported EC volume scrub mode {}",
@@ -4005,6 +4006,36 @@ impl VolumeServer for VolumeGrpcService {
                         }
                     }
                 }
+                4 => {
+                    // CHECKSUM: verify each local shard's raw bytes against the
+                    // bitrot checksum sidecar, exercising cold parity shards.
+                    // Read-only. Mirrors Go's v.ChecksumScrub().
+                    let (blocks_scanned, broken, errs, collection) = {
+                        let store = self.state.store.read().unwrap();
+                        let ecv = store.find_ec_volume(vid).ok_or_else(|| {
+                            Status::not_found(format!("EC volume id {} not found", vid.0))
+                        })?;
+                        let collection = ecv.collection.clone();
+                        let (blocks, broken, errs) = ecv.checksum_scrub();
+                        (blocks, broken, errs, collection)
+                    };
+                    total_volumes += 1;
+                    total_files += blocks_scanned;
+                    if !errs.is_empty() || !broken.is_empty() {
+                        broken_volume_ids.push(vid.0);
+                        for b in broken {
+                            broken_shard_infos.push(volume_server_pb::EcShardInfo {
+                                volume_id: vid.0,
+                                collection: collection.clone(),
+                                shard_id: b,
+                                ..Default::default()
+                            });
+                        }
+                        for msg in errs {
+                            details.push(format!("ecvol {}: {}", vid.0, msg));
+                        }
+                    }
+                }
                 _ => unreachable!(), // validated above
             }
         }