bug: comparison.md claims type preservation that doesn't exist

[27Bslash6]

Description

docs/comparison.md contains claims about cachekit's type handling that are contradicted by the competitive test suite.

Specific Issues

1. The comparison table (line ~46) includes a note about MessagePack tuple→list conversion but frames it as a Redis-only issue. It also happens in L1-only mode (backend=None).

2. Section "Why cachekit wins" for single-process apps claims "Same ~50ns performance (in-memory L1 cache)" without disclosing that L1 mode serializes data (unlike lru_cache which stores raw objects).

3. The "pluggable serializers (v1.0+)" note implies this will be fixed in a future version, but the issue is architectural — L1 serializes regardless of serializer choice.

Evidence

tests/competitive/test_head_to_head.py — 50 tests documenting actual behavior across 4 libraries.

Suggested Fix

Update comparison.md to honestly document:

• cachekit serializes in ALL modes (L1 and L2) for consistency
• This means tuples→lists, frozensets→lists even with backend=None
• The tradeoff: consistent behavior across backends vs type preservation
• cachekit's actual advantages: unhashable args, TTL, async, metrics, upgrade path