docs(forms): add NG01902 error reference and link to docs

Add the NG01902 (Orphan field in signal forms) documentation page
to the Error Encyclopedia and change the ORPHAN_FIELD_PROPERTY
error code to -1902 so Angular's RuntimeError automatically appends
a link to angular.dev/errors/NG01902 in the thrown error message.

adev/src/content/reference/errors/NG01902.md

@@ -0,0 +1,26 @@
+# Orphan field in signal forms
+
+This error indicates that a `Field` instance is no longer connected to the current model structure.
+Angular signal forms throw `NG01902` when a field path that previously existed can no longer be resolved as the same property in the parent object.
+
+A common trigger is unintentionally setting a field's value to `undefined`.
+In signal forms, `undefined` means "field does not exist", so an existing field can become orphaned.
+
+## Why this happens
+
+Signal forms keep field instances in sync with your model shape.
+If code still references an older field instance after the model shape changed, Angular detects the mismatch and throws this error.
+
+This can happen when:
+
+- a property is removed from the model object
+- a property becomes `undefined` instead of remaining present
+- code holds a stale field reference across structural updates
+
+## How to fix it
+
+- Avoid writing `undefined` to model fields. Use `null` or another explicit value when the field should still exist.
+- Keep the model shape stable while a field is in use.
+- Re-read fields from the current form tree after structural updates instead of reusing stale references.
+
+For model design guidance, see [Avoid `undefined`](/guide/forms/signals/model-design#avoid-undefined).

adev/src/content/reference/errors/overview.md

@@ -39,6 +39,7 @@
 | `NG01002` | [Missing Control Value](errors/NG01002)                                              |
 | `NG01101` | [Wrong Async Validator Return Type](errors/NG01101)                                  |
 | `NG01203` | [Missing value accessor](errors/NG01203)                                             |
+| `NG01902` | [Orphan field in signal forms](errors/NG01902)                                       |
 | `NG02200` | [Missing Iterable Differ](errors/NG02200)                                            |
 | `NG02800` | [JSONP support in HttpClient configuration](errors/NG02800)                          |
 | `NG02802` | [Headers not transferred by HttpTransferCache](errors/NG02802)                       |