feat(sve-ui): add Kbd, ButtonGroup, Empty and InputGroup

.changeset/four-small-gaps.md

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+---
+'sve-ui': minor
+---
+
+Four new components: `Kbd`, `ButtonGroup`, `Empty` and `InputGroup`.
+
+These are the first components added since the library finished wrapping every
+one of Bits UI's 41 primitives, so all four are hand-built. They were chosen by
+diffing the catalog against shadcn-svelte: each one is a gap people currently fill
+with a hand-rolled div, and none needs a new dependency.
+
+**`Kbd`** — a keycap. `label` is the part that matters: `⌘`, `⇧` and `⌥` are
+punctuation to a screen reader, and some voices skip them entirely, so a shortcut
+rendered as glyphs alone can be announced as nothing at all. With a label the
+glyph is hidden from the accessibility tree and the word is announced instead.
+The library shipped `Command`, a command palette, with no way to draw the shortcut
+it advertises.
+
+**`ButtonGroup`** — attaches independent buttons into one control, collapsing the
+doubled border between them and lifting whichever has focus so its ring is never
+clipped by a neighbour. `label` or `labelledby` is required: it renders
+`role="group"`, and a role without a name tells a screen reader there is a
+boundary here and then cannot say what for. Use `Toolbar` when the whole bar
+should be one tab stop, and `ToggleGroup` when the buttons carry a value.
+
+**`Empty`** — the state a list, table or result set is in with nothing to show.
+Three defaults, each usually got wrong: `Media` is always `aria-hidden` because
+the icon restates the title; `Title` renders a `<p>` rather than a heading, so an
+empty state inside a card cannot inject "No results" into the document outline;
+and `announce` is opt-in, for the case that actually needs it — results replaced
+by emptiness after a search, which is a change the user caused and cannot see.
+
+**`InputGroup`** — an `Input` with prefix and suffix addons, drawn as one control.
+The group owns the border and the `:focus-within` ring and the input owns neither,
+which is what makes an addon read as part of the field instead of a box beside it.
+`Addon` is `aria-hidden` by default and cannot be the label: a `$` is a hint for
+the eye, and the meaning belongs in the input's accessible name.
+
+Also in this release, both found by tightening a guard rather than by reading code:
+
+- `apps/example`'s coverage guard accepted a namespace member as proof of a
+  top-level export, so `\bEmpty\b` matched `Command.Empty`. With that fixed it
+  reported the standalone `Calendar` as never rendered — true for months, and now
+  rendered on the booking screen. A calendar inside a picker is a different mount.
+- A test asserting `Empty.Root` is not a live region by default was measuring its
+  own fixture's default. Flipping the component default to `true` left it green.
+  Caught by mutation, and the fixture now renders the prop genuinely absent.