feat: checkbox_group carries an accessible name (a11y, #17)

[mhenrixon]

Summary

Closes #17. Follow-up to #9 — checkbox_group's div[role="group"] had no
accessible name, so a screen reader announced "checkbox, Ruby" with no clue the
boxes belong to a "Tags" group. Now the group can carry an accessible name.

Approach (revised after review): no bespoke naming API — attributes pass
through. The leaf forwards extra HTML/ARIA attributes straight to the group
div, so the caller names it with plain aria:. This is smaller and more
flexible than an invented heading:/hint: option.

Two entry points

Bare verb — name it with aria: (a literal string, or an id reference):

f.checkbox_group(:tag_ids, Tag.all, value: :id, aria: { label: "Tags" })
f.checkbox_group(:tag_ids, Tag.all, value: :id, aria: { labelledby: "some_id" })
# → <div role="group" aria-label="Tags">…</div>

Field path — reuses the Control's OWN visible <label>/hint (best practice:
the accessible name matches what sighted users see, no duplicate markup):

f.field(:tag_ids, as: :checkbox_group, collection: Tag.all, value: :id,
        label: "Tags", hint: "Pick any")

<label for="user_tag_ids" id="user_tag_ids_label"><span class="label">Tags</span></label>
<div role="group" aria-labelledby="user_tag_ids_label" aria-describedby="user_tag_ids_hint">…</div>
<p id="user_tag_ids_hint" …>Pick any</p>

The builder stamps stable ids on the Control's label/hint (threaded through
FormControl and Forms::Plain::Control) and passes
aria: { labelledby:, describedby: } through to the group. When no name is
supplied, output is byte-identical to #9 (Phlex omits nil attributes).

Id uniqueness (your Q3)

Field ids derive from scope + name (+ value) exactly like Rails' form_with,
which likewise does not guarantee page-wide uniqueness across multiple forms for
the same model. Documented in the README (scope one form to disambiguate) — not
enforced, matching the Rails contract. No code.

Test coverage

• Bare verb: aria: { label: } → aria-label; aria: { labelledby:/describedby: }
  passthrough; arbitrary data: passthrough; attribute-delimiter escaping (a
  " in the value is "-escaped so it can't break out — </> are inert
  inside a quoted attribute); only role+aria-invalid when no aria given.
• Field path: Control's label/hint get ids, group aria-* point at them, exactly
  one "Tags" text node (no duplicate).
• Theme parity: bare verb under plain (zero styling classes); plain field path
  stamps the Control label/hint ids.

Full suite: 153 examples, 0 failures · rubocop lib spec clean.

Deviations & judgment calls

• Pivoted from an invented API to attribute passthrough (reviewer's steer).
  The first cut added heading:/hint: options that rendered internal <span id>
  elements. On review we agreed plain aria: passthrough is simpler and more
  flexible — the leaf now forwards @attributes.except(:class) to the group and
  owns no naming vocabulary. Dropped the heading/hint spans, their id generation,
  and the heading_classes/hint_classes seams.
• aria-label (string) for the bare verb, aria-labelledby (id) for the field
  path. The field path has a visible label, so pointing at it keeps the
  accessible name in sync with what's on screen; the bare verb has no such
  element, so a literal aria-label string is the pragmatic default (the caller
  can still pass aria: { labelledby: } to reference their own heading).
• Daisy/plain drift, caught mid-implementation: Forms::Plain::Control
  overrides view_template, so it needed the same label_id:/hint_id:
  threading as FormControl — without it the plain field path pointed
  aria-labelledby at a label with no id (a dangling reference). Fixed, with a
  dedicated plain-field-path regression spec.
• Group id: is not settable — the leaf's id: keyword seeds the checkbox
  ids, so a caller can't override the group div's id via id:. Out of scope for
  checkbox_group: group has no accessible name (no legend/aria-labelledby) #17 (naming); noted here so it isn't mistaken for a passthrough gap.
• Id uniqueness: documented, not enforced — matches Rails (your call).