Logs in as the seed user, navigates to the blueprint detail page,
types sv_che into the editor, asserts the autocomplete popup appears
with sv_cheats, accepts via Tab, and asserts the hidden textarea
(form field) now contains the inserted cvar.
This exercises the full chain end-to-end: editor mount on
DOMContentLoaded, srccfg-vocab.json fetch, popup positioning,
capture-phase keydown handling (Task 9 fix), Range-API completion
insertion, and textarea-mirroring on every input.
