feat(headless): add a Button primitive with focusableWhenDisabled

.changeset/hip-moles-jam.md

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+---
+---

packages/headless/package.json

@@ -9,6 +9,10 @@
       "import": "./dist/primitives/accordion/index.js",
       "types": "./dist/primitives/accordion/index.d.ts"
     },
+    "./button": {
+      "import": "./dist/primitives/button/index.js",
+      "types": "./dist/primitives/button/index.d.ts"
+    },
     "./tabs": {
       "import": "./dist/primitives/tabs/index.js",
       "types": "./dist/primitives/tabs/index.d.ts"

packages/headless/src/primitives/button/README.md

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+# Button
+
+A button with the disabled behaviour a native `<button>` cannot express: staying focusable while inert, and behaving like a button on elements that aren't one.
+
+## When to Use
+
+- A button that disables itself mid-interaction — while a form submits, while a request is in flight. Use `focusableWhenDisabled` so focus is not dropped to the body and the user keeps their place on the page.
+- Rendering a button as a link or a `<span>`. Pass `nativeButton={false}` so the role, tab order, and Enter/Space activation are applied.
+- A plain always-enabled button needs neither prop, and a bare `<button>` is fine.
+
+## Usage
+
+```tsx
+import { Button } from '@/primitives/button';
+
+<Button onClick={save}>Save</Button>;
+```
+
+### Focusable while disabled
+
+```tsx
+<Button
+  type='submit'
+  disabled={submitting}
+  focusableWhenDisabled
+>
+  Save
+</Button>
+```
+
+The button keeps its place in the tab order, is marked `aria-disabled`, and ignores clicks and keyboard activation. Focus is not pulled off its current element by a pointer press either.
+
+Suppression is the consumer's handler plus the event's default action, not propagation. Events still bubble, so an enclosing dialog or menu keeps seeing them. Every key but `Tab` has its default prevented — `Tab` is exempt so focus can still move off the button, which is the point of keeping it focusable.
+
+### Non-native element
+
+```tsx
+<Button
+  nativeButton={false}
+  render={<a href='/settings' />}
+>
+  Settings
+</Button>
+```
+
+## Props
+
+| Prop                    | Type                  | Default | Description                                                        |
+| ----------------------- | --------------------- | ------- | ------------------------------------------------------------------ |
+| `disabled`              | `boolean`             | `false` | Makes the button inert                                             |
+| `focusableWhenDisabled` | `boolean`             | `false` | Keeps a disabled button in the tab order                           |
+| `nativeButton`          | `boolean`             | `true`  | Whether the rendered element is a real `<button>`                  |
+| `render`                | `RenderPropOrElement` | —       | Renders a different element (see the shared `render` escape hatch) |
+
+Standard `<button>` attributes pass through. `type` defaults to `"button"` on a native button and is overridable.
+
+## Data Attributes
+
+| Attribute       | Description           |
+| --------------- | --------------------- |
+| `data-disabled` | Present when disabled |
+
+## ARIA
+
+- Native + disabled: the `disabled` attribute.
+- Native + `disabled` + `focusableWhenDisabled`: `aria-disabled="true"`, no `disabled` attribute.
+- `nativeButton={false}`: `role="button"`, `tabIndex={0}` (`-1` when disabled without `focusableWhenDisabled`, since an `<a href>` is tabbable on its own and dropping the attribute would leave it in the tab order), `aria-disabled` when disabled. `Enter` and `Space` activate it; `Enter` on a link is left to the browser so it does not fire twice.