feat: add dependency graph collector with range expansion

[Blackgaurd]

Summary

• Add service/scheduler.py with collect_graph() -- DFS walker that collects all dependency nodes and their required time ranges
• Handles lookback range expansion, start-date clamping, and diamond dependency deduplication (range union + child re-walk)
• Returns a DependencyGraph(ranges, edges) structure for consumption by Kahn's algorithm (next PR)
• Thorough docstrings on every function explaining invariants and non-obvious behavior

Test plan

• 10 unit tests covering: single root, linear chain, diamond dedup, lookback expansion, lookback propagation through chains, diamond with different lookbacks (range union), start-date clamping, end-before-start-date error, lookback clamped by dep start-date, re-expansion propagation to grandchildren
• `just fix` passes
• `just test` passes

🤖 Generated with Claude Code

[Blackgaurd]

• refactor: replace _build_recursive with orchestrator #323 : 2 dependent PRs (#324 , #325 )
• feat: add build orchestrator #322
• feat: add build worker that executes a single job #321
• feat: add topological scheduler with Kahn's algorithm #320
• feat: add dependency graph collector with range expansion #319 👈 (View in Graphite)
• refactor: add job descriptor and build plan types #318
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.