Default renderer for CaravelaSvelte — implements the LiveView
WebSocket transport (today's live_svelte behaviour).
prepare/1 takes the raw assigns that arrive at
CaravelaSvelte.svelte/1 and returns an enriched map with
everything the HEEx template needs:
:init,:svelte_id,:use_diff:slots— decoded for JS consumption:ssr_render— SSR output, ornilwhen disabled / not first-render:props_to_send,:props_diff,:streams_diff
All diff / ID / encoder helpers live here too so that Phase B.2's
CaravelaSvelte.Rest can reuse them without touching top-level
CaravelaSvelte.