feat: add topological scheduler with Kahn's algorithm

[Blackgaurd]

Summary

• Add schedule_build() to service/scheduler.py -- Kahn's algorithm that consumes collect_graph() output and produces a BuildPlan
• Jobs grouped by topological level: level 0 = roots, level N+1 = datasets whose deps all live in levels 0..N
• Thorough docstrings explaining Kahn's algorithm, why it's chosen over Tarjan's (natural level-order output), and barrier semantics

Test plan

• 5 new tests: single root, linear chain ordering, diamond levels, lookback ranges in JobDescriptors, independence within levels
• All 15 scheduler tests pass
• `just fix` passes
• `just test` passes

🤖 Generated with Claude Code

[Blackgaurd]

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• 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 👈 (View in Graphite)
• feat: add dependency graph collector with range expansion #319
• refactor: add job descriptor and build plan types #318
• main

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