Krikri::ResultDisplay::StderrCapture

Fiber-scoped stack of active stderr captures: real prints a task's warnings and deprecations at display time, i.e. only when that task is actually reached, but krikri's batch-build evaluates every group member's when: and prepares its action up front (a template: renders on the controller there) - emitters that fire during that preparation (issue #489's authorized_key jinja-constant warning, an INJECT_FACTS_AS_VARS or play_hosts read in a never-reached member's when: or template, issue #979) would surface ahead of earlier members' output and for tasks that never run. Capturing during the build and re-emitting at consumption restores real's ordering; dropping a never reached member's capture restores real's silence. A stack, like the substitutor's fiber-scoped stacks, so a nested capture keeps its own text; with no capture active the write is byte-identical to a plain STDERR.puts.