* refactor(cli): extract per-target scan into scanTarget
pull the target loop body into scanTarget, which returns a targetScan
holding that target's findings, report rows, scan labels and log files
instead of appending onto run-wide slices. the run loop merges those in
target order and finishRun handles the post-loop notify/silent/report/
summary steps. behavior is identical for the sequential run today; the
isolation is the seam a bounded worker pool needs next.
* feat(cli): scan targets in parallel with -concurrency
add -concurrency N (default 1) to run the per-target scan pool over
several targets at once. scanTarget already returns isolated
accumulators, so workers share only the console: output.SetConcurrent
serializes sink writes and de-animates spinners/progress so parallel
lines stay whole. results merge in input order, and concurrency 1 keeps
the sequential path unchanged.
