feat(fp): WP-16 — component a11y: description wiring + alert role

Wires text-input's aria-describedby to the form-field description div
(the BSN hint was rendered but never announced), pins desc-before-error
ordering, and switches alert to role=alert for errors vs role=status
for info/ok/warning. Composition contract enforced by story play tests
(form-field+text-input, alert per variant) run in the WP-01 CI gate.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/shared/ui/text-input/text-input.component.ts

@@ -1,4 +1,4 @@
-import { Component, forwardRef, input } from '@angular/core';
+import { Component, booleanAttribute, forwardRef, input } from '@angular/core';
 import { ControlValueAccessor, NG_VALUE_ACCESSOR } from '@angular/forms';
 
 /** Atom: text input. Utrecht textbox wired up as a form control (ngModel/reactive). */
@@ -22,7 +22,7 @@ import { ControlValueAccessor, NG_VALUE_ACCESSOR } from '@angular/forms';
       [type]="type()"
       [id]="inputId()"
       [attr.aria-invalid]="invalid() ? 'true' : null"
-      [attr.aria-describedby]="invalid() && inputId() ? inputId() + '-error' : null"
+      [attr.aria-describedby]="describedBy()"
       [placeholder]="placeholder()"
       [disabled]="disabled"
       [value]="value"
@@ -39,12 +39,24 @@ export class TextInputComponent implements ControlValueAccessor {
   placeholder = input('');
   invalid = input(false);
   inputId = input<string>();
+  /** Set when the paired form-field renders a `-desc` hint, so it gets announced. */
+  hasDescription = input(false, { transform: booleanAttribute });
 
   value = '';
   disabled = false;
   onChange: (v: string) => void = () => {};
   onTouched: () => void = () => {};
 
+  describedBy(): string | null {
+    const id = this.inputId();
+    if (!id) return null;
+    const ids = [
+      ...(this.hasDescription() ? [`${id}-desc`] : []),
+      ...(this.invalid() ? [`${id}-error`] : []),
+    ];
+    return ids.length ? ids.join(' ') : null;
+  }
+
   onInput(e: Event) {
     this.value = (e.target as HTMLInputElement).value;
     this.onChange(this.value);