Repair the child-tracker unit tests against the current API (unit
ChildSignal::Registered, single-argument OrchestrationChildTracker::new),
fold their setup into a shared fixture, and give each test a single
behavioral invariant. Rewrite comments across the M1-changed files to
explain why rather than how, dropping spec, milestone, and prior-state
references.

Co-Authored-By: Oz <oz-agent@warp.dev>