refactor: add job descriptor and build plan types

[Blackgaurd]

Summary

• Add JobDescriptor, JobResult, and BuildPlan dataclasses in new service/models.py
• Pure data types for the upcoming scheduler/queue/worker refactor
• JobDescriptor is frozen/hashable for use as dict keys
• BuildPlan.levels groups jobs by topological depth (roots first, requested dataset last)

Test plan

• 12 unit tests covering equality, hashing, immutability, and structural properties
• 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
• refactor: add job descriptor and build plan types #318 👈 (View in Graphite)
• main

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