feat(conflict): Init Tango-backed target-overlap conflict analyzer

[manjari25]

Summary

• Adds a conflict.Analyzer implementation that detects conflicts between
  batches based on build-target overlap, resolved through a TargetResolver
  interface whose production implementation will call Tango's
  GetChangedTargets in a follow-up PR.
• The analyzer is the same shape as pathoverlap — takes identity-level
  batches, resolves targets internally, compares sets locally — but operates
  at build-target granularity instead of file paths.
• YARPC adapter and profile wiring follow in subsequent PRs.

Test plan

• Table-driven tests covering overlap, no overlap, ordering, empty candidate targets
• Empty in-flight short-circuits with no conflicts
• Resolver error propagation
• make test, make lint, make fmt, make check-gazelle pass

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[behinddwalls]

should this be an internal interface?