Simplify runnable naming and add FuncNamed

[pior]

• Simplify naming: Remove findName chain-walking, baseWrapper struct,
  and RunnableName/RunnableUnwrap implicit interfaces. Wrappers now
  compose a name string at construction time using a single unexported
  runnableName() helper with reflection fallback.
• Add FuncNamed(name, fn): Lets users name function runnables explicitly,
  solving the problem of multiple Func() instances being indistinguishable
  in logs.
• Add ManagerName option: Customize the manager's name in logs
  (defaults to "manager").
• Func() uses runtime.FuncForPC: Auto-detects meaningful function names
  instead of showing the generic type name.
• Update README: Fix stale Manager(nil) → NewManager(), update log
  output examples to slog format.