Function render_partial 

pub fn render_partial(
    protocol_data: Buffer,
    state_json: String,
    entry_id: String,
    request_path: String,
    inventory_hex: String,
) -> Result<String>

Produce a complete JSON partial response for client-side navigation.

Combines application state, route templates, inventory, request path, and matched route chain into a single JSON string: {"state":{...},"templates":[...],"inventory":"...","path":"...","chain":[...]}.

Host servers return this directly — no assembly required.