fix(print): handle svelte:body and fix keyframe percentage double-printing (#18234)

Fixes #18206 and fixes #18207 — both are printer bugs in
`packages/svelte/src/compiler/print/index.js`.

## Changes

### Fix 1: `svelte:body` crashes the printer (#18206)

`SvelteBody` was missing from the visitor map, causing a crash with
`Error: Not implemented: SvelteBody`. Added the handler (same one-liner
pattern as `SvelteDocument`, `SvelteHead`, etc.) and added `SvelteBody`
to the `is_block_element` check so whitespace is handled consistently.

### Fix 2: Keyframe percent stops print as `0%%` (#18207)

`Percentage.value` already includes the `%` sign (captured by
`/\d+(\.\d+)?%/y`), but the printer was writing `` `${node.value}%` `` —
appending a second `%`. Changed to `context.write(node.value)` to match
the `Nth` printer pattern directly above it.

Also updated the existing `style` snapshot which had `50%%` (the bug was
silently baked in), and added dedicated test samples for both fixes.