Replace ad-hoc `Vec<SLAResult>` history read with cursor envelope

**Problem Statement.** `get_history_page(offset, limit)` lacks a "next"
cursor — clients must compute offsets themselves.

**Expected Outcome.**
- New fn: `get_history_page_cursor(after: Option<u32>, limit: u32) -> HistoryPage`.
- Returns cursor + entries.

**Files:** `lib.rs`, tests.