fix: high memory usage

[Ryder-C]

Problem

Seadexerr was using around 230 MiB of memory at all times, mostly because it deserialized way more of the mapping file than it needed to.

Changes

• Parse mappings.json into borrowed keys with the values discarded (IgnoredAny) instead of building a full serde_json::Value tree (over 75k+ unnecessary keys)
• Reuse a single reference-counted buffer for the parse and the disk write instead of holding two copies.
• Cap the Tokio worker pool to 2.

Result

~230 MiB → ~39 MiB at idle