Pure bank_grid gains hit-test, selection-update, and arrow-nav math with
tests; the panel wires mouse multi-select, keyboard nav via an accelerator
hook, and stock PlayPreview/StopPreview audition with a leak-free preview
lifecycle. Read-only: no arrange insertion, no project/bank mutation.
Docked SWELL window toggled by a new action, drawing the current bank as
per-sample min/max thumbnails (PCM_source + peaks) with an in-memory cache.
Pure grid-layout/cache-key math in bank_grid (tested). Renames peaks::Sample
-> AudioSample to avoid colliding with bank_model::Sample.
daniel