Replace unmaintained bincode with postcard serialization

bincode has been marked as unmaintained (RUSTSEC-2025-0141). This commit
replaces it with postcard, which:
- Has an API compatible with bincode (serialize/deserialize)
- Is actively maintained by James Munns with 114+ libraries
- Offers comparable performance and better binary size
- Is well-suited for the RPC parameter passing use case

Changes:
- Update Cargo.toml files: bincode -> postcard with use-std feature
- Replace bincode::serialize with postcard::to_stdvec
- Replace bincode::deserialize with postcard::from_bytes
- Update error type conversions from bincode::Error to postcard::Error
- Rename internal types (BincodeValue -> PostcardValue) for clarity

All tests pass except for unrelated bottomless integration test.

Author: Drew Robinson

Cargo.lock

@@ -29,7 +29,7 @@ cbc = "0.1.2"
 
 [dev-dependencies]
 arbitrary = { version = "1.3.0", features = ["derive_arbitrary"] }
-bincode = "1.3.3"
+postcard = { version = "1", features = ["use-std"] }
 tempfile = "3.8.0"
 prost-build = "0.12"
 tonic-build = "0.11"

libsql-replication/Cargo.toml

@@ -18,7 +18,7 @@ async-trait = "0.1.58"
 axum = { version = "0.6.18", features = ["headers"] }
 axum-extra = { version = "0.7", features = ["json-lines", "query"] }
 base64 = "0.21.0"
-bincode = "1.3.3"
+postcard = { version = "1", features = ["use-std"] }
 bottomless = { version = "0", path = "../bottomless", features = ["libsql_linked_statically"] }
 bytes = { version = "1.2.1", features = ["serde"] }
 bytesize = { version = "1.2.0", features = ["serde"] }

libsql-server/Cargo.toml

@@ -244,8 +244,8 @@ impl From<tokio::sync::oneshot::error::RecvError> for Error {
     }
 }
 
-impl From<bincode::Error> for Error {
-    fn from(other: bincode::Error) -> Self {
+impl From<postcard::Error> for Error {
+    fn from(other: postcard::Error) -> Self {
         Self::Internal(other.to_string())
     }
 }