refactor(supervisor): make managed child ownership structural

Description

Make Linux managed-child ownership structural so callers cannot accidentally separate spawning, PID registration, waiting, unregistering, and lost-child diagnostics.

Context

The child reaper race fixed by PR #3142 required every Linux child path to use the correct registration protocol. The current registry helper closes the immediate spawn-to-registration window, but ownership remains a convention across canonical and SSH process paths. The diagnostics and wait-handling follow-ups explored during that review belong with the ownership cleanup.

Definition of Done

• Introduce a managed-child abstraction that owns Linux child registration and cleanup.
• Route canonical and SSH child paths through the abstraction.
• Centralize explicit-wait handling, unregistering, and ECHILD diagnostics.
• Add deterministic tests for ownership and reaper interleavings.
• Preserve non-Linux behavior without introducing Linux-only reaper machinery there.

Related Work

• PR #3142

Start by reading PR #3142, then trace the canonical and SSH child paths named in the issue to understand their current registration and cleanup flow. Done means both paths use one managed-child abstraction, wait and ECHILD handling are centralized, deterministic interleaving tests exist, and non-Linux behavior remains unchanged.