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.

tp-form.js

@@ -7,6 +7,22 @@ This program is available under Apache License Version 2.0
 import { LitElement, html, css } from 'lit';
 import { clone } from '@tp/helpers/clone.js';
 
+// Events that indicate a form control's value changed. Fired either by
+// tp-* controls (see the control's source for the exact name) or natively
+// (input/change from wrapped native fields). Listened to in the capture
+// phase so events that don't bubble (e.g. tp-color-picker's 'change') are
+// caught as well.
+const CHANGE_EVENTS = [
+  'input',
+  'change',
+  'value-changed',
+  'selection-changed',
+  'checked-changed',
+  'toggled',
+  'number-changed',
+  'slider-changed',
+];
+
 class TpForm extends LitElement {
   static get styles() {
     return [
@@ -78,6 +94,7 @@ class TpForm extends LitElement {
     super();
     this._origValues = [];
     this._wasSetReadonly = [];
+    this._onControlChanged = this._onControlChanged.bind(this);
 
     this._addElement = this._addElement.bind(this);
     this._removeElement = this._removeElement.bind(this);
@@ -101,6 +118,11 @@ class TpForm extends LitElement {
     this.addEventListener('form-element-register', this._addElement);
     this.addEventListener('form-element-unregister', this._removeElement);
 
+    // Notify about control changes via `form-changed` — see CHANGE_EVENTS.
+    for (const name of CHANGE_EVENTS) {
+      this.addEventListener(name, this._onControlChanged, true);
+    }
+
     // Holds all custom elements registered.
     this._controls = [];
   }
@@ -109,6 +131,10 @@ class TpForm extends LitElement {
     super.disconnectedCallback();
     this.removeEventListener('form-element-register', this._addElement);
     this.removeEventListener('form-element-unregister', this._removeElement);
+
+    for (const name of CHANGE_EVENTS) {
+      this.removeEventListener(name, this._onControlChanged, true);
+    }
   }
 
   _handleSlotChange(e) {
@@ -351,10 +377,46 @@ class TpForm extends LitElement {
   }
 
   /**
-   * Check if some form control's value was changed.
+   * True once snapshot() was called at least once. Dirty checks are only
+   * meaningful against a baseline.
+   */
+  get hasSnapshot() {
+    return this._snapshot !== undefined;
+  }
+
+  /**
+   * Check if some form control's value was changed since the last
+   * snapshot(). Before the first snapshot() there is no baseline to
+   * compare against, so this reports false instead of comparing against a
+   * fabricated empty form.
    */
   isChanged() {
-    return JSON.stringify(this.serialize()) !== (this._snapshot || '{}');
+    if (!this.hasSnapshot) {
+      return false;
+    }
+
+    return JSON.stringify(this.serialize()) !== this._snapshot;
+  }
+
+  /**
+   * True if el is a control registered with this form (custom control or
+   * unwrapped native element). Scopes form-changed to the form's own
+   * controls when forms are nested.
+   */
+  _isRegisteredControl(el) {
+    return this._controls.includes(el) || Array.from(this._nativeElements).includes(el);
+  }
+
+  _onControlChanged(e) {
+    if (!this._isRegisteredControl(e.target)) {
+      return;
+    }
+
+    this.dispatchEvent(new CustomEvent('form-changed', {
+      detail: { changed: this.isChanged() },
+      bubbles: true,
+      composed: true,
+    }));
   }
 
   /**