isChanged() compared against a fabricated '{}' baseline before the first
snapshot() call, so the first query on any non-empty form reported true.
Consumers that embed the boolean in their own baseline (settings dirty
guards) froze that false positive in. Now reports false until a baseline
exists; hasSnapshot getter exposes the state.
New: form listens (capture phase, so non-bubbling events are caught) to
the tp-* control change events and re-emits them as 'form-changed'
(detail.changed = isChanged()), scoped to its own registered controls.
Consumers can drop polling.
Use a dollar sign to indicate where tp-form should collect values as an
array. For example `myfield.$list.value` collects { myfield: { list: [ {
value: '...' } ] } }