feat(ec): remove the .ecsum sidecars when destroying an EC volume

remove_ec_volume_files now clears <base>.ecsum (and any versioned .ecsum.v<N>)
from the data and idx dirs, so a vid reuse can't load a stale sidecar. Mirrors
Go's removeBitrotSidecars.

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

seaweed-volume/src/storage/disk_location.rs

@@ -395,6 +395,12 @@ impl DiskLocation {
         for i in 0..MAX_SHARD_COUNT {
             rm_if_present(format!("{}.ec{:02}", base, i))?;
         }
+
+        // Remove the bitrot checksum sidecars from both the data and idx dirs.
+        remove_bitrot_sidecars(&base)?;
+        if self.idx_directory != self.directory {
+            remove_bitrot_sidecars(&idx_base)?;
+        }
         Ok(())
     }
 
@@ -1081,6 +1087,49 @@ fn rm_if_present(path: String) -> io::Result<()> {
     }
 }
 
+/// Remove the bitrot checksum sidecars for a base file name: the legacy
+/// `<base>.ecsum` (generation 0) and any versioned `<base>.ecsum.v<N>`.
+/// Already-gone is success; returns the first real removal failure (and surfaces
+/// a directory-scan error) so a stale sidecar left behind is not reported as
+/// cleaned. Mirrors Go's removeBitrotSidecars.
+fn remove_bitrot_sidecars(base: &str) -> io::Result<()> {
+    use crate::storage::erasure_coding::ec_bitrot::BITROT_SIDECAR_EXT;
+    let rm = |path: std::path::PathBuf| -> io::Result<()> {
+        match fs::remove_file(&path) {
+            Err(e) if e.kind() != io::ErrorKind::NotFound => Err(e),
+            _ => Ok(()),
+        }
+    };
+    let mut first_err: Option<io::Error> = None;
+    let mut record = |res: io::Result<()>| {
+        if let Err(e) = res {
+            if first_err.is_none() {
+                first_err = Some(e);
+            }
+        }
+    };
+    record(rm(format!("{}{}", base, BITROT_SIDECAR_EXT).into()));
+    let path = std::path::Path::new(base);
+    if let (Some(parent), Some(fname)) = (path.parent(), path.file_name()) {
+        let prefix = format!("{}{}.v", fname.to_string_lossy(), BITROT_SIDECAR_EXT);
+        match fs::read_dir(parent) {
+            Ok(entries) => {
+                for entry in entries.flatten() {
+                    if entry.file_name().to_string_lossy().starts_with(&prefix) {
+                        record(rm(entry.path()));
+                    }
+                }
+            }
+            Err(e) if e.kind() != io::ErrorKind::NotFound => record(Err(e)),
+            Err(_) => {}
+        }
+    }
+    match first_err {
+        Some(e) => Err(e),
+        None => Ok(()),
+    }
+}
+
 fn ec_data_shards_from_vif(directory: &str, idx_directory: &str, collection: &str, vid: VolumeId) -> usize {
     for dir in [directory, idx_directory] {
         let vif = format!("{}.vif", volume_file_name(dir, collection, vid));