[M10/LNS-001] Extract DAG utilities into src/dag.js

[flyingrobots]

LNS-001: Extract DAG Utilities

Parent: #227 (M10 LENS)

What

Extract DAG utilities from src/views.js (lines 305-446) into src/dag.js:

• Adjacency builder (from edge list → adjacency maps)
• Topological sort (Kahn's algorithm)
• Cycle detection
• Chain walking (follow edges from a start node)
• Root finding (nodes with no incoming edges)

Why

These utilities are needed by lenses (LNS-004) and are currently buried in views.js. Extracting them makes them composable and testable in isolation.

Acceptance Criteria

• src/dag.js exports: buildAdjacency, topoSort, detectCycles, walkChain, findRoots
• src/views.js imports from src/dag.js (no duplication)
• All existing view tests still pass
• New unit tests for each DAG utility
• No dependencies on graph internals — pure functions on edge arrays

Dependencies

None — can start immediately.