The last missing piece connecting the backend preset mechanism to something
a person can actually click: pipeline.js gets a presets() fetcher matching
load/describe/check/search's own shape, and pipeline_window.js's group-card
index offers a "Load preset..." button (only when a caller passes one) that
lists what /api/pipeline/presets returns and replaces the pipeline through
the same commit()/check() path every other structural edit already goes
through -- a preset that does not actually build is refused here, same as
a hand-drawn bad wire would be, never shown as if it had loaded.
Writing the test for this caught a real bug before it shipped: open()'s own
destructuring silently dropped `presets` before ever reaching mount(), so
the button would never have appeared no matter what boot.js passed it.
digital-garbage
33e2b02f4d