feat(fortal): add FortalCheckboxGroupItem and share the focus ring

[leoafarias]

Closes the last missing Fortal generated component, and de-duplicates the focus ring while in the same code.

No remix API was touched.

1. FortalCheckboxGroupItem

RemixCheckboxGroupItem was the only Remix widget owning a Styler with no Fortal recipe. The reason it was orphaned is structural:

Item	Self-rendering widget?	Parent has styler?	Parent spec has item slot?
RemixToggleGroupItem	no (data)	yes	yes
RemixSegmentedControlItem	no (data)	yes	yes
RemixSelectItem	no (data)	yes	yes
RemixMenuItem	no (data)	yes	yes
RemixCheckboxGroupItem	yes	no	no

Every other item is a data class whose parent spec carries an item slot the Fortal parent recipe fills. RemixCheckboxGroup is behavioral and has no styler, so nothing pushed item styling down — callers hand-attached the recipe to each item, and a missed one in a loop rendered unstyled beside its styled siblings.

Before / after

// before — the recipe has to be attached to every item by hand
final _checkboxStyle = fortalCheckboxStyle();

RemixCheckboxGroup<String>(
  values: _channels,
  onChanged: (v) => setState(() => _channels = v),
  child: Wrap(children: [
    for (final (value, label) in options)
      RemixCheckboxGroupItem<String>(
        value: value,
        label: label,
        style: _checkboxStyle,   // <- miss one and it renders unstyled
      ),
  ]),
)

// after
RemixCheckboxGroup<String>(
  values: _channels,
  onChanged: (v) => setState(() => _channels = v),
  child: Wrap(children: [
    for (final (value, label) in options)
      FortalCheckboxGroupItem<String>(value: value, label: label),
  ]),
)

fortalCheckboxGroupItemStyle delegates to fortalCheckboxStyle rather than restating any of it, so it inherits the checkbox family's verified parity and introduces no new visual surface. That is also why no new Chromium probe was needed — the reference fixture's 25 probes already cover checkbox.

Parity is unchanged. The Radix checkbox_group family stays in unmappedUpstreamFamilies: the root gap is still caller-owned, which is what the manifest's reopenCondition is about. Only the supportedRemixComposition prose was updated to name the new wrapper, keeping the per-item style escape hatch documented.

2. Shared focus ring

fortalFocusRing / fortalFocusRingBox replace four hand-rolled copies in accordion, toggle, toggle group, and tabs. Strictly behavior-preserving — verified per call site against git show HEAD.

Tabs keeps its solid focus-8 where the other three use alpha focus-a8. That difference is preserved, not unified: the pinned Chromium probes capture computed styles only, not :focus-visible, so the reference cannot settle whether it is intentional. A test fails if someone unifies it, forcing the decision to be deliberate.

Two entry points because FlexBoxStyler is a Mix type that sits outside Remix's RemixBoxStylerAnchors interface — a compiler-enforced boundary, not a style choice.

Rendered output

Every component below is the real recipe, rendered at 2x with Roboto loaded — not a mock. Checkbox group is the row this PR adds.

Usage for each family shown above

// Wrap the app (or any subtree) once. Everything below reads from this scope.
FortalScope(accent: .indigo, gray: .slate, child: MyApp());

// Actions — variant is a named constructor or the `variant:` argument
FortalButton(onPressed: () {}, label: 'Button');
FortalButton.soft(onPressed: () {}, label: 'Button');
FortalIconButton(onPressed: () {}, icon: Icons.add, semanticLabel: 'Add');
FortalToggle(selected: true, icon: Icons.format_bold, label: 'Bold', onChanged: (_) {});

// Display
FortalBadge(label: 'Active');
FortalAvatar(label: 'LF', size: .size3);
FortalAvatar.solid(icon: Icons.person);
FortalCallout(icon: Icons.info_outline, text: 'Changes apply across the workspace.');
FortalProgress(value: 0.68);
FortalDataList(items: [
  RemixDataListItem(label: 'Plan', value: 'Enterprise'),
  RemixDataListItem(label: 'Seats', value: '48'),
]);

// Selection
FortalCheckbox(selected: true, label: 'Weekly digest', onChanged: (_) {});
FortalSwitch(selected: true, onChanged: (_) {}, semanticLabel: 'Notifications');
RemixRadioGroup<int>(groupValue: 1, onChanged: (_) {}, child: FortalRadio<int>(value: 1));

// Checkbox group — the API this PR adds
RemixCheckboxGroup<String>(
  values: selected,
  onChanged: (next) => setState(() => selected = next),
  child: Row(children: const [
    FortalCheckboxGroupItem<String>(value: 'email', label: 'Email'),
    FortalCheckboxGroupItem<String>(value: 'sms', label: 'SMS'),
    FortalCheckboxGroupItem<String>(value: 'push', label: 'Push'),
  ]),
);

// Exclusive choice
FortalSegmentedControl<String>(
  selectedValue: 'week',
  onChanged: (v) => setState(() => range = v),
  items: const [
    RemixSegmentedControlItem(value: 'day', label: 'Day'),
    RemixSegmentedControlItem(value: 'week', label: 'Week'),
    RemixSegmentedControlItem(value: 'month', label: 'Month'),
  ],
);

// Input
FortalTextField(hintText: 'Search…');
FortalTextArea(hintText: 'Notes…');

Re-scope to restyle any subtree without touching the widget — this is how the dashboard renders status badges:

FortalScope(
  accent: .red,
  hasBackground: false,
  child: FortalBadge(label: 'Refunded'),
);

Tests

• test/fortal/focus_ring_test.dart — pins all four rings to their pre-refactor literals.
• test/components/checkbox/checkbox_group_widget_test.dart — delegation across the full variant × size × highContrast matrix, plus group-selection plumbing.

The matrix test compares resolved specs rather than stylers, because CheckboxStyler is not value-comparable — see the note below.

Verification

Gate	Result
remix_fortal tests	281 pass
remix tests	2552 pass
dashboard tests	25 pass
analyze (apps + packages)	clean
fortal:parity:check	25 mapped, 3 extensions, 1 audited unmapped
docs:check	pass
generated drift	27 artifacts byte-for-byte

Follow-ups found while doing this (not in this PR)

• RemixToggle does not forward excludeSemantics, so it cannot express a selected destination #119 — RemixToggle does not forward excludeSemantics.
• CheckboxStyler is the only Fortal recipe that is not value-comparable. Root cause is not codegen — the generated props/== are correct. CheckboxStyler.onIndeterminate (packages/remix/lib/src/components/checkbox/checkbox_style.dart:10-18) allocates a fresh ContextVariant with a closure on every call, and ContextVariant has no == override, unlike its subclass WidgetStateVariant that onSelected/onFocused/onDisabled use. Bisected: onSelected, onFocused, onDisabled all preserve equality; only onIndeterminate breaks it. accordion_style.dart:73,85,97,109 has the same latent pattern.
• fortalFocusOutline second pass — checkbox, card, radio, and switch recipes each inline the CSS-outline pattern the existing helper already produces.