feat: add 'pythonic' serializer alias and tuple preservation

[27Bslash6]

Description

Two related improvements to the serializer system:

1. Add serializer='pythonic' alias for AutoSerializer

auto is vague — "auto-detect what?" pythonic communicates the value: preserves Python-native types.

# Current
@cache(serializer='auto')

# Proposed alias
@cache(serializer='pythonic')  # clearer intent

One-line change in SERIALIZER_REGISTRY:

SERIALIZER_REGISTRY = {
    "auto": AutoSerializer,
    "pythonic": AutoSerializer,  # alias — preserves Python types
    ...
}

2. Add tuple preservation to AutoSerializer

AutoSerializer already preserves sets and frozensets via type markers ({"__set__": True, ...}). Tuples need the same treatment:

# In _auto_default():
if isinstance(obj, tuple):
    return {"__tuple__": True, "value": list(obj)}

# In _auto_object_hook():
if obj.get("__tuple__") is True:
    return tuple(obj["value"])

Why

• Sets are preserved, tuples are not — inconsistent
• serializer='auto' (or 'pythonic') would then preserve tuples, sets, frozensets, datetime, UUID, Decimal
• Default serializer='default' stays MessagePack for cross-SDK compatibility
• The tradeoff is explicit: pythonic = Python-only type fidelity, default = cross-language safe

Evidence

From tests/competitive/test_head_to_head.py:

• AutoSerializer preserves sets through cache roundtrip ✓
• AutoSerializer does NOT preserve tuples through cache roundtrip ✗ (same MessagePack issue as default)
• ~10 LOC fix in auto_serializer.py

References: #73