feat(cache): add pluggable TTL caching layer to GuildPassClient

[AlAfiz]

Ran command: git checkout -b feat/ttl-cache-layer
Ran command: git commit -m "feat(cache): add pluggable TTL caching layer to GuildPassClient"
Ran command: git push -u origin feat/ttl-cache-layer

## feat(cache): Pluggable TTL Caching Layer for GuildPassClient

### Overview
Implements an optional, transparent caching layer for `GuildPassClient` that memoizes all safe read operations without changing any public API signatures.

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### What Changed

**`src/cache/cache.types.ts`** *(new)*
- `CacheAdapter` — generic interface (`get<T>`, `set<T>`, `delete`, `clear`) designed to be implemented by any backend (in-memory, Redis, etc.)
- `InMemoryCacheAdapter` — zero-dependency default implementation backed by a `Map` with `Date.now()`-based TTL eviction

**`src/config/sdkConfig.ts`**
- Added `cache?: CacheAdapter` and `cacheTtl?: number` to `GuildPassClientConfig`

**`src/client/GuildPassClient.ts`**
- When a `cache` adapter is provided, each read service (`access`, `membership`, `roles`, `guilds`) is replaced at construction with a prototype-delegating proxy (`Object.create`), keeping every method signature identical to callers
- Deterministic composite cache keys (e.g. `access:checkAccess:{guildId}:{resourceId}:{walletAddress}`)
- New public invalidation helpers: `invalidateGuildCache(guildId)`, `invalidateWalletCache(walletAddress)`, `clearCache()` — all no-ops when no adapter is configured

**`src/index.ts`**
- Re-exports `CacheAdapter` and `InMemoryCacheAdapter` from the package root

**`tests/cache.test.ts`** *(new)*
- 16 tests across two suites covering: cold misses, typed cache hits, TTL expiry via `vi.useFakeTimers()`, no-TTL persistence, delete/clear, guild-scoped invalidation, no-adapter safety, and custom adapter injection (Redis stub pattern)

**`README.md`**
- New **⚡ Caching** section documenting `InMemoryCacheAdapter`, a complete Redis adapter example, and all three invalidation helpers
- Removed the now-shipped roadmap item

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### Key Design Decisions
- **Zero breaking changes** — caching is entirely opt-in; omitting `cache` from config preserves existing behaviour exactly
- **Adapter pattern** — decouples the SDK from any specific cache implementation; Redis, Memcached, or any async store can be plugged in with ~15 lines
- **No new runtime dependencies** — `InMemoryCacheAdapter` uses only native `Map` and `Date.now()`

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### Testing

Tests 16 passed (16)

All TTL assertions use fake timers — no real delays in the test suite.

closes #15