fix(js): implement HTMLInputElement.indeterminate

The property was never defined anywhere in the engine, so
`'indeterminate' in checkbox` was false and scripts could neither set nor
read the flag. Add it as an IDL-only property backed by a node-keyed store,
the same way `checked` is stored, since element wrappers are rebuilt on
every lookup.

Checkbox activation now clears it too. HTML's legacy-pre-activation
behavior toggles checkedness and sets indeterminateness to false, and the
canceled path restores both. Two copies of that logic needed the change:
`Element#click()` in bootstrap.js, and the pre-click activation snippet in
obscura-cdp that drives real dispatched mouse input.

Fixes #732.