refactor: drop redundant intermediate allocations

[Mimori256]

Summary

Remove unnecessary heap allocations in three independent locations. The most impactful is engine_serialize, which was cloning the entire serialized payload before handing it to the JS layer; the other two eliminate redundant intermediate Strings on code paths that are correct without them.

src/blocker.rs

Pass QParam directly to itertools::join instead of mapping through .to_string(). QParam already implements Display, and itertools::join formats elements via Display, so the per-parameter intermediate String was unnecessary. Saves one allocation per query parameter on every removeparam URL rewrite.

src/cosmetic_filter_utils.rs

Replace key += &char.to_string() with key.push(char) when appending a decoded \XXXX escape codepoint. String::push writes the char's UTF-8 bytes directly into the existing buffer, removing the per-escape-sequence String allocation.

js/src/lib.rs

Engine::serialize() already returns an owned Vec<u8> (see src/engine.rs and serialize_dat_file in src/data_format/mod.rs, which builds a fresh Vec to prepend the magic+hash header). The extra .to_vec() was producing a full duplicate of the entire serialized engine. Remove it and let JsArrayBuffer::copy_from_slice do the only required copy (Rust heap → JS heap). The MutexGuard is still released at the end of the if let arm; this is safe because the returned Vec<u8> owns its data.