feat: add inline getOrLoad API

[lan17]

Summary

• add loader-first DialCache.getOrLoad(load, options) and exported GetOrLoadOptions<Value> for one-shot inline cache blocks
• route both cached() and getOrLoad() through one internal cache-execution primitive while preserving stable-use-case registration semantics
• document direct-key/captured-value requirements and cover local, Redis, invalidation, coalescing, fail-open, type, and packed-package behavior

Why

cached(fn, options) is ergonomic for extracted reusable functions, but immediately invoking a newly wrapped function is awkward for inline work and conflicts with unique useCase registration. getOrLoad() keeps the loader inline, permits repeated calls with one bounded stable useCase, and preserves the existing cache pipeline.

Impact

This is an additive public API. Existing callers, cache-key bytes, Redis data and scripts, invalidation identity, adapters, metrics labels, dependency ownership, and cached() duplicate-registration behavior are unchanged.

Closes #71

Validation

• corepack pnpm check on Node 22.22.0: strict types, 261 unit tests with coverage, ESM/CommonJS builds and declarations, packed runtime/declaration consumers
• corepack pnpm test:package on Node 20.17.0
• corepack pnpm test:integration: 69 live Redis 6.2, Valkey 8, node-redis, Valkey GLIDE, and Redis Cluster cases
• corepack pnpm benchmark:request-local: all semantic assertions passed
• corepack pnpm audit --prod --audit-level high: no known vulnerabilities
• git diff --check