test(core): guard shared-terminal child output against double rendering

[NathanFlurry]

• Add a brush-interactive regression cell: an external wasm command (cat staged as childcat, spawned as a child of the shell) must render its output exactly once in the terminal. Guards the secure-exec fix for shared-terminal child output being emitted twice (doubled cat lines, corrupted vim keystroke echo).
• The cell skips when the local registry cat artifact is absent, matching the suite's existing fixture gate.