Problem:
- Original code used `let sanitized = {}` which never garbage collects
- This causes unbounded memory growth in SSR scenarios
- Heap analysis showed ~5,400 concatenated strings per feed generation

Solution:
- Replace plain object with LRUCache from lru-cache package
- Cap at 10,000 entries (~2 feed generations worth)
- Count-based eviction with 5 MB safety cap
- Maintains string primitives (no String object overhead like WeakMap approach)

Based on heap analysis:
- ~21,500 strings/minute growth rate
- ~5,400 strings per feed (15s regeneration cycle)
- Average string size: ~32 bytes
- 10,000 entries = ~320 KB stable memory footprint

Related: developit#20, developit#34, developit#43

- Replace LRU cache with WeakMap approach from PR developit#43
- Use String objects as WeakMap keys for automatic garbage collection
- Memory bounded to active rendering context, no manual management needed
- Update tests to use valueOf() to convert String objects to primitives

Fixes developit#20, developit#34
Based on: developit#43

- Replace unbounded cache with WeakMap
- Return String objects instead of primitives
- String objects can be WeakMap keys and are GC'd automatically
- No eviction issues - entries only released when truly unreferenced
- Requires calling .valueOf() on final result before serialization

This approach is based on PR developit#43 but adapted for server-side usage.