test(scenarios): cover rig capture with a prefix-overlap stub scenario (#612)

Adds a stub scenario for rig capture with two seats whose member names overlap by prefix (dev-impl and dev-impl2). Each seat's stub script prints its own token, and each expect step captures one full seat name through the shipped rig capture --json and requires that seat's token, so a capture that resolves the seat by member-name prefix or by first match fails at the intended step. A capture that returns both panes is not detected, because contains has no absence form. The scenario uses env.stub_scripts, so it runs locally in host mode; the installed-scenario CI job and container mode do not execute it, like the transcript scenario. test-layers.md now counts 15 scenarios and 7 topologies, separates the scenarios that run in CI from those run locally, and lists the capture row's remaining gaps.