Add JetStream KV bucket support (v2.1.0)

[majkelx]

Summary

Adds first-class support for NATS JetStream KV buckets to the Messenger, following the existing driver architecture.

New driver hierarchy (all under MsgDriver):

MsgKvDriver                # common base: bucket bind/create, envelope encode/decode
├── MsgKvStore             # put / get / delete / purge / keys / history / status
└── MsgKvReader            # async iterator over kv.watch(), yields (data, meta)
    └── MsgKvSubscriber    # callback on each change, stop-on-False semantics

• Full envelope semantics: values are always serverish {"data", "meta"} envelopes — put validates against schemas, reading a foreign (non-serverish) entry raises MessengerKvMalformed instead of returning garbage. The only protocol-forced exception: KV delete/purge tombstones carry no payload and surface as (None, meta) with meta['kv']['operation'] set to 'DEL'/'PURGE'.
• Key/revision metadata in meta['kv']: bucket, key, revision, operation, created.
• ConnectionJetStream.ensure_kv_bucket() — buckets are not created on the fly by default (consistent with stream philosophy); drivers opt in via create_bucket=True + bucket_config.
• Factories: Messenger.get_kvstore / get_kvreader / get_kvsubscriber, module-level get_kvstore/get_kvreader/get_kvsubscriber and one-shot kv_put / kv_get.
• New exceptions: MessengerKvBucketNotFound, MessengerKvKeyNotFound, MessengerKvMalformed.
• The watch loop lives only in MsgKvReader, the background-task machinery only in MsgKvSubscriber — linear hierarchy prepared for a future MsgKvDocumentReader (LiveDocument over a bucket).

Version bumped to 2.1.0 (new feature, backward compatible).

Testing

• 9 new tests in tests/test_messenger_kv.py (put/get round-trip, delete, keys/history, malformed entry, missing bucket, one-shot helpers, watch iteration incl. delete markers, callback subscriber).
• Full suite: 300 passed, 8 skipped, 1 xfailed — no regressions.

Related: #34 (shared Event() default in MsgCallbackSubscriber, noticed while implementing MsgKvSubscriber).

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https://claude.ai/code/session_01HfMGwatJe81U9aekr51R7f