rust volume: bump redb 3.1.3 -> 4.2.0 (#11181)

No source changes: the API surface the needle map uses (Database
create/open/builder, set_cache_size, set_durability, tables, iterators)
is unchanged and the on-disk format is still v3, so existing .rdb files
open as-is. The 4.0.0 breaking changes (Drop on AccessGuardMut, removal
of the Legacy type) do not touch this crate.

Relevant to the redb-backed index (#11179):
- 4.1.0: optimizes cache usage and memory usage; ~1.5x faster writes.
- 4.2.0: Durability::None commits ~2x faster; pages freed by a durable
  transaction are reused by the very next one; a crash-recovery fix for
  a crash during repair of an earlier crash.


Claude-Session: https://claude.ai/code/session_019x36FiSeyePh77YXao15kK

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

seaweed-volume/Cargo.lock

@@ -3160,9 +3160,9 @@ dependencies = [
 
 [[package]]
 name = "redb"
-version = "3.1.3"
+version = "4.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4ba239c1c1693315d3cc0e601db3b3965543afbf48c41730fdca2f069f510f4a"
+checksum = "de6c3b63e007e90ce536ec2ae4690826136a20ec8dbbbb400daef1bb999d2e36"
 dependencies = [
  "libc",
 ]

seaweed-volume/Cargo.toml

@@ -57,7 +57,7 @@ rustls-pemfile = "2"
 rusty-leveldb = "3"
 
 # Disk-backed needle map (alternative to in-memory HashMap)
-redb = "3"
+redb = "4"
 
 # Reed-Solomon erasure coding
 reed-solomon-erasure = "6"