The check/uncheck/select branches of browser_fill_form were left on the
pre-#530 pattern: a direct `el.checked = true` / `el.selectedIndex = i` plus a
raw, untrusted `change` event. For a React-controlled checkbox the direct
assignment runs the framework's value tracker in lockstep, so onChange never
commits and the box reverts; and any handler gating on event.isTrusted ignores
the untrusted events -- the exact bug class #530 fixed for text inputs two
lines away.
Route check/uncheck through __obscura_setFieldValue(el, 'checked', ...) so the
write goes through the prototype setter and leaves the tracker stale, and
dispatch input+change wrapped in __obscura_markTrusted for all three branches,
matching the text-input branch.
Fixes#550
58c7fe0543