fix(seaweed-volume): accept redb aliases for --index (#9237)

fix(seaweed-volume): accept redb aliases for --index and rename kinds

The Rust volume server's disk-backed index uses redb internally
(see RedbNeedleMap), but --index only accepted the legacy `leveldb`
spellings, contradicting the wiki and forcing users to read source to
figure out what value to pass.

- --index now accepts memory|redb|redbMedium|redbLarge as the canonical
  names, with leveldb/leveldbMedium/leveldbLarge kept as aliases.
- Rename NeedleMapKind variants LevelDb*->Redb* so the in-tree names
  match the actual backend.
- Update help text and add a parse-table test covering both names.

Refs #9234.

seaweed-volume/src/storage/needle_map.rs

@@ -146,9 +146,9 @@ impl NeedleMapMetric {
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum NeedleMapKind {
     InMemory,
-    LevelDb,
-    LevelDbMedium,
-    LevelDbLarge,
+    Redb,
+    RedbMedium,
+    RedbLarge,
 }
 
 // ============================================================================

seaweed-volume/src/storage/volume.rs

@@ -776,7 +776,7 @@ impl Volume {
     fn load_index(&mut self) -> Result<(), VolumeError> {
         let use_redb = matches!(
             self.needle_map_kind,
-            NeedleMapKind::LevelDb | NeedleMapKind::LevelDbMedium | NeedleMapKind::LevelDbLarge
+            NeedleMapKind::Redb | NeedleMapKind::RedbMedium | NeedleMapKind::RedbLarge
         );
 
         let idx_path = self.file_name(".idx");