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>

docs/backlog/WP-16-component-a11y.md

@@ -1,6 +1,6 @@
 # WP-16 — Component a11y: description wiring + alert role
 
-Status: todo
+Status: done (pending commit)
 Phase: 4 — a11y
 
 ## Why
@@ -57,12 +57,25 @@ Audit findings axe can't (fully) catch:
 
 ## Acceptance criteria
 
-- [ ] Description text is programmatically associated in the canonical composition;
+- [x] Description text is programmatically associated in the canonical composition;
       login-form BSN hint announced.
-- [ ] `-error` id appended exactly when invalid; order stable.
-- [ ] Error alerts are `role="alert"`; play tests assert both behaviors and run in the
+- [x] `-error` id appended exactly when invalid; order stable.
+- [x] Error alerts are `role="alert"`; play tests assert both behaviors and run in the
       CI gate.
 
+## Deviation from the original plan
+
+`radio-group`/`checkbox` were left unchanged — grepping every call site found zero
+consumers pairing either with a `form-field` `description` (only the login-form BSN
+field, which uses `text-input`). The WP's own Files note ("describedby joins; only
+where the component takes a hint") already carved out this exact case — adding
+`hasDescription` to atoms with no live description consumer would be unused surface,
+not a fix. `radio-group` already had correct `-error`-only wiring; untouched.
+`describedBy()` was added directly on `text-input` rather than factored into a shared
+`shared/kernel` helper — one consumer, ~5 lines, not worth the indirection yet.
+Manual screen-reader spot check (optional per the WP) skipped; the play test is the
+enforced check going forward.
+
 ## Verification
 
 GREEN + `npm run test-storybook:ci` (includes the new play tests).