fix(decode): preserve mapKeyConverter in Decoder#clone() - #295

Problem

Decoder#clone() is used for re-entrant decoding — for example an extension codec that decodes nested MessagePack on the same reusable Decoder instance (the pattern from #195). clone() copies every decoder option except mapKeyConverter, so any map decoded re-entrantly silently falls back to the default key converter instead of the one the caller configured.

Repro (converter upper-cases keys; extension type 1 re-enters the same decoder):

const decoder = new Decoder({ extensionCodec, mapKeyConverter: (k) => String(k).toUpperCase() });
// extension type 1 decode: (data) => new Wrapped(decoder.decode(data))
const out = decoder.decode(encoder.encode({ a: 1, nested: new Wrapped({ b: 2 }) }));
// top-level keys: ["A", "NESTED"]   (converter applied)
// out.NESTED.inner keys: ["b"]      (converter dropped — should be ["B"])

Fix

Copy mapKeyConverter in clone() alongside the other options (keyDecoder and the max* limits are already copied). One line, plus a regression test that decodes a nested map through an extension codec.

Full suite: 328 passing; lint and typecheck clean.