feat(core): join safe html fragments

Let already-safe fragments compose without downgrading to String, and use it in the v0 page assembly example to keep rendered and lowered fragments typed at the boundary.

req: html_safety/001

req: html_safety/002

req: component/003

REQUIREMENTS.md

@@ -561,7 +561,7 @@ what a valid business email is.
 001	Raw HTML insertion requires an explicit safe HTML type (`SafeHtml` or equivalent). Plain `String` renders as escaped text unless explicitly wrapped.
 
 ### req: html/002
-002	Hemplate-rendered output may be converted to `SafeHtml` by trusted render APIs. User input is never `SafeHtml` by default. Full-page shell composition may pass already-rendered hemplate fragments through explicit `SafeHtml` fields; handlers should prefer slot/resource render helpers for effect payloads.
+002	Hemplate-rendered output may be converted to `SafeHtml` by trusted render APIs. User input is never `SafeHtml` by default. Full-page shell composition may pass already-rendered hemplate fragments through explicit `SafeHtml` fields, and already-safe fragments may be joined without downgrading to `String`; handlers should prefer slot/resource render helpers for effect payloads.
 
 ### req: html/003
 003	Slot render commands distinguish text payloads from HTML payloads at the type level.