[M10/LNS-004] Implement 5 core lenses

[flyingrobots]

LNS-004: Implement 5 Core Lenses

Parent: #227 (M10 LENS)

What

Implement 5 core lenses using DAG utilities from LNS-001:

1. incomplete — filter to nodes missing status:done/status:closed (needs properties)
2. frontier — filter to leaf nodes (no outgoing edges in the filtered set)
3. critical-path — filter to longest dependency chain (uses topoSort + chain walking)
4. blocked — filter to nodes with unresolved incoming blocks edges
5. parallel — filter to nodes with no mutual dependency (can execute concurrently)

Why

These are the most immediately useful projections for task/project management workflows.

Acceptance Criteria

• Each lens registered via defineLens from LNS-002
• Each lens uses DAG utilities from src/dag.js (LNS-001)
• incomplete correctly checks node properties for status
• frontier finds leaves in the filtered subgraph
• critical-path returns the longest chain
• blocked checks blocks edge type specifically
• parallel identifies independent nodes
• Comprehensive tests for each lens with fixture graphs
• Edge cases: empty graph, single node, cycles (critical-path should handle gracefully)

Dependencies

• LNS-001 (DAG utilities)
• LNS-002 (lens registry)

[flyingrobots]

Implemented in M10 LENS milestone. Closing as done.