Define isChanged() before first snapshot + emit form-changed on control changes

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.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@tp/tp-form",
-  "version": "1.2.1",
+  "version": "1.3.0",
   "description": "",
   "main": "tp-form.js",
   "scripts": {