[config] Named presets — fast/balanced/accurate profiles

[dgenio]

Context

Split from #45. Adds named presets for quick configuration without manual tuning.

Proposal

Add ProfileConfig.from_preset(name) classmethod supporting "fast", "balanced", and "accurate" presets:

Preset	beam_width	max_depth	top_k	budget.answer
fast	1	4	5	3000
balanced	2	8	10	6000
accurate	4	12	20	8000

profile = ProfileConfig.from_preset("fast")
router = Router(graph, items=catalog.all(), **profile.routing_kwargs())

Acceptance Criteria

• ProfileConfig dataclass in config.py bundling: budget, policy, scoring, routing (a new RoutingConfig dataclass)
• RoutingConfig dataclass: beam_width, max_depth, top_k, confidence_gap, max_children
• ProfileConfig.from_preset(name) supporting "fast", "balanced", "accurate"
• "balanced" preset produces identical behavior to current defaults
• to_dict() / from_dict() on both dataclasses
• Backward-compatible: all existing constructors unchanged
• Tests for all 3 presets and roundtrip serialization

File Paths

• src/contextweaver/config.py (edit)
• tests/test_config.py (edit)

Parent Issue

Part of #45 (ProfileConfig).