refactor: replace _build_recursive with orchestrator

[Blackgaurd]

Wires run_build() into build_dataset() and deletes _build_recursive(). This is the switchover PR.

What changed

• build_dataset() now delegates to run_build() (orchestrator) instead of _build_recursive()
• Deleted _build_recursive() (~120 lines)
• Moved NoValidTimestampsError and generate_timestamps to new service/timestamps.py to break circular import (builder -> orchestrator -> worker -> builder)
• builder.py re-exports both for backward compatibility (api/routes.py, tests)
• Updated test_builder.py: replaced 16 tests that tested _build_recursive internals with 3 delegation tests (behavior now covered by test_scheduler + test_worker + test_orchestrator)
• Updated test_concurrent_builds.py: mock targets changed from service.builder.* to service.scheduler.registry + service.worker.*

Why

_build_recursive mixed scheduling (dependency resolution, start-date clamping, lookback expansion) with execution (timestamp check, builder subprocess, DB insert). The new architecture separates these concerns into scheduler, worker, and orchestrator.

Benefit

• Clear separation: scheduler computes what to build, worker builds one dataset, orchestrator coordinates
• All 302 tests pass (net -10 tests from deduplication, not lost coverage)
• Integration tests unchanged
• Concurrent build tests still exercise real locking through the full pipeline

🤖 Generated with Claude Code

[Blackgaurd]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• refactor: replace _build_recursive with orchestrator #323 : 2 dependent PRs (#324 , #325 ) 👈 (View in Graphite)
• 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
• main

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