feat: intent-based cache API (CacheKit::minimal, ::production, ::secure, ::io)

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Context

Python and TypeScript SDKs now have intent-based factory functions that pre-configure the full cache stack from a single declarative call:

Python: @cache.minimal(), @cache.production(), @cache.secure(), @cache.io()
TypeScript: createCache.minimal(), .production(), .secure(), .io() (cachekit-io/cachekit-ts#1)

The Rust SDK is the only one missing this pattern.

Proposal

Add intent-based builder methods:

// Speed-first — no circuit breaker, minimal config
let cache = CacheKit::minimal("redis://localhost:6379")
    .ttl(300)
    .build()?;

// Reliability-first — circuit breaker + retry
let cache = CacheKit::production("redis://localhost:6379")
    .ttl(600)
    .build()?;

// Zero-knowledge encryption
let cache = CacheKit::secure("redis://localhost:6379")
    .master_key(key)
    .build()?;

// SaaS backend
let cache = CacheKit::io()
    .api_key("ck_live_...")
    .ttl(3600)
    .build()?;

Intent Specifications

Intent	Backend	Circuit Breaker	Retry	L1 SWR	Encryption	Default TTL
minimal	Redis	Off	Off	Off	No	300s
production	Redis	On (threshold: 5)	On	On	No	600s
secure	Redis	On (threshold: 5)	On	On	AES-256-GCM	600s
io	cachekit.io	On (threshold: 5)	On	On	Optional	3600s

References

• Python implementation: cachekit-py/src/cachekit/decorators/intent.py
• TypeScript implementation: cachekit-ts/packages/cachekit/src/intents.ts
• SDK feature matrix: protocol/sdk-feature-matrix.md