chore: add changeset for host-overridable slots

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: benvinegar

.changeset/host-overridable-slots.md

@@ -0,0 +1,13 @@
+---
+"sideshow": minor
+---
+
+Add host-overridable slots to the embeddable viewer engine. Two layout regions
+that carry deployment-specific guidance — the empty-board onboarding and the
+sidebar footer's instructional links — are now wrapped in named `<slot>`s whose
+fallback content is the existing self-hosted markup. An embedder projects
+light-DOM children with a matching `slot=` attribute into the mount element to
+replace a whole region; with nothing projected (and self-hosted, outside a
+shadow root) the fallback renders unchanged, so self-hosted parity is preserved.
+The new `SLOTS` registry and `SlotName` type are exported from the embed entry
+and `embed.d.ts` so embedders share one typed source of truth.