feat(cache): add unified Cache

[tomas-zijdemans]

NOTE: Builds on this PR

This PR replaces LruCache and TtlCache with a single Cache<K, V> that unifies LRU eviction, TTL expiration, stale-while-revalidate, and load-through (getOrLoad).

The existing caches extend Map, so inherited methods bypass LRU/TTL logic entirely. Both subclass Map, making it structurally impossible to compose LRU with TTL.

Cache<K, V> uses composition over inheritance: it owns a Map for storage and delegates to IndexedHeap for deadline-ordered expiration. Mode is determined by options, not class choice, and a discriminated union on CacheOptions makes illegal combinations compile-time errors.

Beyond fixing the structural issues, this introduces capabilities that didn't exist before:

• Combined LRU + TTL in a single cache
• getOrLoad(key, loader) with automatic in-flight deduplication
• Stale-while-revalidate with background refresh, error handling, and configurable soft/hard TTL thresholds
• Per-entry TTL overrides and sliding expiration
• Single heap-driven timer replacing per-entry setTimeouts
• Hit/miss/eviction stats, typed ejection reasons, and Symbol.dispose

Since @std/cache is still experimental (0.2.2), this is the right time to make a clean break before stabilization locks things in.

[codecov]

Codecov Report

❌ Patch coverage is 99.50577% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.46%. Comparing base (1cd63ca) to head (820fbf9).

Files with missing lines	Patch %	Lines
cache/cache.ts	99.32%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7082      +/-   ##
==========================================
+ Coverage   94.41%   94.46%   +0.05%     
==========================================
  Files         630      630              
  Lines       50490    50876     +386     
  Branches     8949     9041      +92     
==========================================
+ Hits        47669    48060     +391     
+ Misses       2249     2243       -6     
- Partials      572      573       +1     

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