perf: replace JSON+base64 cache envelope with compact binary framing

[matthewdevenny]

Problem

Every cached value is wrapped in a JSON CacheEntry (src/NatsDistributedCache/NatsCache.cs), whose Data is a byte[]. System.Text.Json serializes byte[] as base64 — roughly 33% size inflation — plus a JSON serialize/parse on every Set/Get.

Investigation

The natural "store raw bytes + put expiration in NATS message headers" approach is blocked on the read path: NatsKVEntry<T> exposes no Headers (only Bucket/Created/Delta/Error/Key/Operation/Revision/Value), so header metadata can't be recovered through the standard KV read API without dropping to raw JetStream.

Proposal

Replace the JSON envelope with a custom INatsSerialize<CacheEntry> / INatsDeserialize<CacheEntry> using compact binary framing, e.g.:

[version:1][flags:1][absExpTicks:8][slidingExpTicks:8][raw payload …]

No base64, no JSON; metadata stays in the value (no dependency on header reads). A leading version byte allows future format evolution.

Acceptance criteria

• Stored size ≈ payload + small fixed header (no base64).
• Absolute + sliding expiration round-trip correctly.
• version byte present and checked on read.
• Benchmark demonstrates reduced bytes and allocations vs. the current JSON path.

Notes / risk

Breaking on-wire format change. Bump the format version and document that pre-existing entries should be treated as misses (or add a one-version read shim for a transition window). This is the biggest single efficiency lever and deserves its own design pass.