[19/24] feat: add supervision macros

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Summary

• Add child_spec! macro for creating child specifications:

  child_spec!(worker "counter", Counter::new(0).start())
  child_spec!(supervisor "sub_sup", SubSupervisor::start(spec))

• Add supervisor_spec! macro for creating supervisor specifications:

  supervisor_spec!(OneForOne, [
      child_spec!(worker "w1", Worker::new().start()),
      child_spec!(worker "w2", Worker::new().start()),
  ])
  
  supervisor_spec!(OneForAll, max_restarts(5, 60), [
      child_spec!(worker "db", Db::new().start()),
  ])

• Add dynamic_supervisor_spec! macro for dynamic supervisors:

  dynamic_supervisor_spec!()
  dynamic_supervisor_spec!(max_children(50), max_restarts(10, 60))

• Add strategy() getter to SupervisorSpec
• Add get_max_children() getter to DynamicSupervisorSpec

Test plan

• All 88 tests pass (9 new macro tests)
• Clippy passes

🤖 Generated with Claude Code

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Reorganizing PR sequence for coherent implementation order. This content will be included in the new PR series.