Maestro compat: support `childOf` on assertVisible/assertNotVisible (parity with tapOn)

[thymikee]

Context

childOf is supported on tapOn but rejected on assertVisible/assertNotVisible. The asymmetry is arbitrary rather than decided: assertions ride the same target-resolution path that already implements childOf scoping for taps (scopeMaestroMatchesByAncestor).

Repro

• Corpus flow: scripts/maestro-conformance/corpus/upstream/114_child_of_selector.yaml
• Declared divergence: FLOW_DIVERGENCES['upstream/114_child_of_selector'] in scripts/maestro-conformance/expected-divergence.ts

Acceptance

• Parser accepts childOf on both assertion commands; resolution scopes by ancestor exactly as tapOn does (first-match semantics preserved).
• 114 divergence entry removed; verifier classifies identical; conformance green with no new divergences.