fix(filter): bump recursion_limit to 4096 to fix lib test E0275

ruvector-filter lib test build hits a recursion overflow evaluating
`&mut Vec<u8>: std::io::Write` (required for serde_json's Serializer
impl). The previous limit of 2048 was insufficient on stable rustc as
of 2026-04; the compiler explicitly suggests 4096. Bumping resolves
the core-and-rest test shard failure.

Co-Authored-By: claude-flow <ruv@ruv.net>

crates/ruvector-filter/src/lib.rs

@@ -1,4 +1,4 @@
-#![recursion_limit = "2048"]
+#![recursion_limit = "4096"]
 
 //! # rUvector Filter
 //!